DOT HS 811 154 August 2009

NHTSA Tire Fuel Efficiency Consumer Information Program Development: Phase 2 Effects of Tire Rolling Resistance Levels on Traction, Treadwear, and Vehicle Fuel Economy

This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161

This publication is distributed by the U.S. Department of Transportation, National Highway TrafficSafetyAdministration, in the interestof informationexchange.Theopinions,findingsand conclusions expressed in this publication are those of the author(s) and not necessarily those of the Department of Transportation or the National Highway Traffic Safety Administration. The United States Government assumes no liability for its content or use thereof. If trade or manufacturers names or products are mentioned, it is because they are considered essential to the object of the publication and should not be construed as an endorsement. The United States Government does not endorse products or manufacturers.

TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No. DOT HS 811 154

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle NHTSA Tire Fuel Efficiency Consumer Information Program Development: Phase 2 Effects of Tire Rolling Resistance Levels on Traction, Treadwear, and Vehicle Fuel Economy

5. Report Date August 2009 6. Performing Organization Code

7. Author(s) Larry R. Evans,1 James D. MacIsaac Jr.,2 John R. Harris,1 Kenneth Yates,2 Walter Dudek,1 Jason Holmes,1 Dr. James Popio,3 Doug Rice,3 Dr. M. Kamel Salaani1

1Transportation Research Center, Inc., 2National Highway Traffic Safety Administration, 3Smithers Scientific Services, Inc.

8. Performing Organization Report No.

9. Performing Organization Name and Address National Highway Traffic Safety Administration Vehicle Research and Test Center P.O. Box B-37 10820 State Route 347 East Liberty, OH 43319-0337

10. Work Unit No. (TRAIS)

11. Contract or Grant No. DTNH22-03-D-08660, DTNH22-07-D-00060

12. Sponsoring Agency Name and Address National Highway Traffic Safety Administration 1200 New Jersey Avenue SE. Washington, DC 20590

13. Type of Report and Period Covered Final 14. Sponsoring Agency Code NHTSA/NVS-312

15. Supplementary Notes Project support and testing services provided by: NHTSA San Angelo Test Facility, Akron Rubber Development Laboratory, Inc., Smithers Scientific Services, Inc., Standards Testing Laboratories, Inc., and Transportation Research Center, Inc. 16. Abstract This report summarizes the second phase of the project to develop a tire fuel efficiency consumer information program intended to examine possible correlations between tire rolling resistance levels and service variables such as vehicle fuel economy, wet and dry traction, and outdoor and indoor treadwear. Tires of 15 different models with known rolling resistances were installed on the same new passenger car to evaluate their effects of on vehicle fuel economy. A 10percent decrease in tire rolling resistance resulted in an approximately 1.1-percent increase in fuel economy for the vehicle. This result was within the range predicted by technical literature. Reducing the inflation pressure by 25 percent resulted in a small but statistically significant decrease of approximately 0.3 to 0.5 miles per gallon for four of the five fuel economy cycles, excluding the high-speed, high-acceleration US06 cycle. This value was smaller than many values predicted by technical literature, and possible explanations are being explored.

Tires of 16 different models with known rolling resistances were subjected to dry and wet skid-trailer testing on asphalt and concrete skid pads. Both the peak (maximum) and slide (fully locked-tire) coefficients of friction were measured and indexed against the control tire. For the tires studied, there appeared to be no significant relationship between dry peak or slide numbers and rolling resistance. However, these tire models exhibited a strong and significant relationship between better rolling resistance and poorer wet slide numbers. The peak wet slide number displayed the same tendency, but the relationship was much weaker. This may be significant to consumers without anti-lock braking systems (ABS) on their vehicles since the wet slide value relates most closely to locked-wheel emergency stops. For newer vehicles with ABS or electronic stability control systems, which operate in the earlier and higher wet peak friction range, the tradeoff is less significant. For the subset of 5 tire models subjected to on-vehicle treadwear testing (UTQGS), no clear relationship was exhibited between tread wear rate and rolling resistance levels. For the subset of 6 tire models subjected to significant amounts of wear in the indoor treadwear tests, there was a trend toward faster wear for tires with lower rolling resistance. This report concludes with an analysis of the various options in the draft ISO 28580 rolling resistance test and their likelihood of inducing variability in the test results, as well as a discussion of data reporting format. 17. Key Words Tire, rolling resistance, consumer information, tire traction, Energy Independence and Security Act of 2007 (EISA)

18. Distribution Statement This report is free of charge from the NHTSA Web site at www.nhtsa.dot.gov

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No. of Pages 153

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

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http://www.nhtsa.dot.gov/

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Approximate Conversions to Metric Measures

Symbol When You Know Multiply by To Find Symbol

LENGTH

in inches 2.54 centimeters cm ft feet 30 centimeters cm mi miles 1.6 kilometers km

AREA

in2 square inches 6.5 square centimeters cm2 ft2 square feet 0.09 square meters m2 mi2 square miles 2.6 square kilometers km2

MASS (weight)

oz ounces 28 grams g

lb pounds 0.45 kilograms kg

PRESSURE

psi pounds per inch2 0.07 bar bar psi pounds per inch2 6.89 kilopascals kPa

VELOCITY

mph miles per hour 1.61 kilometers per hour km/h

ACCELERATION

ft/s2 feet per second2 0.30 meters per second2 m/s2

TEMPERATURE (exact)

F Fahrenheit 5/9 (Celsius) - 32C Celsius C

Approximate Conversions to English Measures Symbol When You Know Multiply by To Find Symbol

LENGTH

mm millimeters 0.04 inches in cm centimeters 0.4 inches in m meters 3.3 feet ft km kilometers 0.6 miles mi

AREA

cm2 square centimeters 0.16 square inches in2 km2 square kilometers 0.4 square miles mi2

MASS (weight)

g grams 0.035 ounces ozkg kilograms 2.2 pounds lb

PRESSURE

bar bar 14.50 pounds per inch2 psi kPa kilopascals 0.145 pounds per inch2 psi

VELOCITY

km/h kilometers per hour 0.62 miles per hour mph

ACCELERATION

m/s2 meters per second2 3.28 feet per second2 ft/s2

TEMPERATURE (exact) C Celsius 9/5 (Celsius) + 32F Fahrenheit F

TABLE OF CONTENTS

1.0 INTRODUCTION ................................................................................................................................ 1

1.1 THE CONCEPT OF ROLLING RESISTANCE .................................................................................... 3 2.0 METHODOLOGY............................................................................................................................... 7

2.1 TEST TIRES .................................................................................................................................. 7 2.1.1 ASTM F2493 Radial Standard Reference Test Tire ............................................................... 7

2.2 TIRE ROLLING RESISTANCE TEST PROCEDURES ......................................................................... 8 2.2.1 ISO Draft International Standard 28580 Single-Point Rolling Resistance ........................... 11 2.2.2 SAE J1269 & ISO 18164 Multi-Point Rolling Resistance.................................................... 11 2.2.3 SAE J2452 Multi-Point (Speed Coast Down) Rolling Resistance........................................ 11

2.3 FUEL ECONOMY TEST VEHICLE ................................................................................................ 11 2.4 TEST WHEELS ............................................................................................................................ 11 2.5 TEST MATRIX ............................................................................................................................ 12 2.6 TREAD COMPOUND PROPERTIES TESTING................................................................................. 13 2.7 ON-VEHICLE FUEL ECONOMY TESTING .................................................................................... 16

2.7.1 EPA 40 CFR Part 86 Dynamometer Fuel Economy Testing ................................................ 17 2.8 SKID-TRAILER TIRE TRACTION TESTING .................................................................................. 21 2.9 ON-VEHICLE TIRE TREADWEAR TESTING ................................................................................. 23 2.10 INDOOR TIRE TREADWEAR TESTING ......................................................................................... 25

3.0 RESULTS............................................................................................................................................ 28 3.1 EFFECT OF TIRE ROLLING RESISTANCE ON AUTOMOBILE FUEL EFFICIENCY........................... 28

3.1.1 Preliminary Analysis: Data Shifts ......................................................................................... 30 3.1.2 Highway FET Triplicate Analysis: ....................................................................................... 31 3.1.3 Air Conditioning SC03 11/20/08 to 11/25/08 .................................................................... 34 3.1.4 Analysis by Date for Possible Drift in Data over Time ........................................................ 35 3.1.5 Effect of Tire Rolling Resistance on Fuel Economy............................................................. 37 3.1.6 Effect of Reduced Inflation Pressure on Fuel Economy ....................................................... 43 3.1.7 Fuel Economy Testing Summary.......................................................................................... 50

3.2 CORRELATION OF TANGENT AT 60C TO TIRE ROLLING RESISTANCE .................................. 51 3.3 EFFECT OF TIRE ROLLING RESISTANCE ON SAFETY.................................................................. 53

3.3.1 Dry Traction Data ................................................................................................................. 53 3.3.2 Wet Traction Data ................................................................................................................. 56 3.3.3 UTQGS Traction Grade ........................................................................................................ 59 3.3.4 Correlation of Tangent at 0C to Wet Traction Properties................................................. 61

3.4 EFFECTS OF TIRE ROLLING RESISTANCE ON TREADWEAR RATE.............................................. 62 3.4.1 Analysis of Wear Data From Indoor Treadwear Testing ...................................................... 65

4.0 CONCLUSIONS................................................................................................................................. 78 5.0 REQUIREMENTS ............................................................................................................................. 79 6.0 ROLLING RESISTANCE (Fr) VERSUS ROLLING RESISTANCE COEFICIENT Cr)......... 85

6.1 THEORY OF FR AND CR.............................................................................................................. 85 6.1.1 Using Cr from a Single-Load Test to Predict Rolling Resistance at Any Load.................... 90

6.2 DISCUSSION ............................................................................................................................... 94

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Appendix 1. Tire and Rim Association, Inc. - Maximum Load Formula for P Type Tires..... 100 Appendix 2. Detailed Test Matrix ..................................................................................................... 101 Appendix 3. Examples of Data Acquired From Indoor Treadwear Test ...................................... 103 Appendix 4. Raw Dry Traction Testing Results - Asphalt.............................................................. 124 Appendix 5. Raw Dry Traction Testing Results - Concrete ........................................................... 126 Appendix 6. Raw Wet Traction Testing Results - Asphalt ............................................................. 128 Appendix 7. Raw Wet Traction Testing Results - Concrete........................................................... 130 Appendix 8. UTQG Adjusted Wet Traction Testing Results ......................................................... 132 Appendix 9. ASTM E501 Reference Tire Wet Traction Testing Results ...................................... 134 Appendix 10. ASTM E501 Reference Tire Dry Traction Testing Results .................................. 135

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LIST OF FIGURES

Figure 1. Where Does the Energy Go? ........................................................................................... 4

Figure 2. Contribution of Tire Rolling Resistance to Vehicle Fuel Economy Versus Speed......... 5

Figure 3. Magic Triangle: Traction, Treadwear, and Rolling Resistance....................................... 6

Figure 4. Force Method Rolling Resistance Test Machine........................................................... 10

Figure 5. Torque Method Rolling Resistance Test Machine ........................................................ 10

Figure 6. Sample TGA Weight Loss Curve.................................................................................. 14

Figure 7. Tan as a Function of Temperature From the Tension Test ........................................ 15

Figure 8. Tan as a Function of Temperature From the Shear Test ............................................ 16

Figure 9. Vehicle Fuel Economy Dynamometer Testing ............................................................. 18

Figure 10. NHTSA San Angelo Skid-Trailer ............................................................................... 22

Figure 11. UTQGS Treadwear Course ......................................................................................... 25

Figure 12. Indoor Treadwear Equipment...................................................................................... 27

Figure 13. Vehicle Fuel Economy Dynamometer Exhaust Collection Bags and Control System32

Figure 14. Highway FET Schedule Fuel Economy Versus Bag Collection Number................... 32

Figure 15. Air Conditioning SC03 Fuel Economy Versus Tire Rolling Resistance by Analysis

Group .................................................................................................................................... 35

Figure 16. Rolling Resistance of Tires Tested Versus Day of Testing......................................... 36

Figure 17. Highway FET (Bag #1) Mileage Versus Tire Rolling Resistance .............................. 39

Figure 18. Highway FET (Bag #2) Mileage Versus Tire Rolling Resistance .............................. 39

Figure 19. Highway FET (Bag #3) Mileage Versus Tire Rolling Resistance .............................. 40

Figure 20. City FTP Mileage Versus Tire Rolling Resistance ..................................................... 40

Figure 21. High Speed US06 Mileage Versus Tire Rolling Resistance ....................................... 41

Figure 22. Air Conditioning SC03 Mileage Versus Tire Rolling Resistance............................... 41

Figure 23. Cold City FTP Mileage Versus Tire Rolling Resistance............................................. 42

Figure 24. Percentage Change in Fuel Economy Versus Percentage Change in.......................... 43

Figure 25. Tire to Dynamometer Roller Contact / 2008 Chevrolet Impala LS Engine ................ 46

Figure 26. Highway FET (Bag #1) Fuel Economy by Tire Type and Inflation Pressure............. 47

Figure 27. Highway FET (Bag #2) Fuel Economy by Tire Type and Inflation Pressure............. 47

Figure 28. Highway FET (Bag #3) Fuel Economy by Tire Type and Inflation Pressure............. 48

Figure 29. City FTP Fuel Economy by Tire Type and Inflation Pressure.................................... 48

Figure 30. High Speed US06 Fuel Economy by Tire Type and Inflation Pressure...................... 49

Figure 31. Air Conditioning SC03 Fuel Economy by Tire Type and Inflation Pressure ............. 49

Figure 32. Cold City FTP Fuel Economy by Tire Type and Inflation Pressure ........................... 50

Figure 33. Highway FET (Bag #2) Fuel Economy Versus Tire Rolling Resistance by Tire Type

and Inflation Pressure ........................................................................................................... 51

Figure 34. ISO 28580 Rolling Resistance (lbs)Versus Tangent at 60C by Tire Type ............. 52

Figure 35. Dry Traction Numbers Versus ISO 28580 Rolling Resistance ................................... 55

Figure 36. Dry Traction Ratios to E501 Course Monitoring Tire Versus Rolling Resistance ..... 56

Figure 37. Wet Traction Numbers Versus ISO 28580 Rolling Resistance................................... 58

Figure 38. Wet Traction Ratios to E501 Course Monitoring Tire Versus Rolling Resistance..... 59

Figure 39. UTQG Adjusted Traction Coefficient for Asphalt Versus ISO 28580 Rolling

Resistance ............................................................................................................................. 60

Figure 40. UTQG Adjusted Traction Coefficient for Concrete Versus ISO 28580 Rolling

Resistance ............................................................................................................................. 61

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Figure 41. Slide Traction Number on Wet Concrete Versus Tangent at 0C Measured in Tension.................................................................................................................................. 62

Figure 42. Projected Tire Mileage to Wearout (Average and Minimum) Versus ISO 28580

Rolling Resistance ................................................................................................................ 64

Figure 43. Average and Fastest Treadwear Rate Versus ISO 28580 Rolling Resistance............. 65

Figure 44. Projected Tire Lifetime for Indoor Treadwear Test .................................................... 67

Figure 45. Treadwear Rate for Indoor Treadwear Test ................................................................ 68

Figure 46. Projected Tire Lifetime for Indoor Treadwear Test .................................................... 73

Figure 47. ISO 28580 Rolling Resistance Versus Tire Weight Loss ........................................... 74

Figure 48. Rolling Resistance as Percent of the Original Rolling Resistance .............................. 75

Figure 49. Percentage of Original Rolling Resistance.................................................................. 77

Figure 50. Temperature Correction Factor - ISO 28580............................................................... 83

Figure 51. Drum Diameter Correction Factor - ISO 28580.......................................................... 84

Figure 52. SAE J1269 Recommended Test - Evaluates Response of Rolling Resistance Force

Over a Range of Three Pressures and Two Loads................................................................ 87

Figure 53. ISO 18164 Annex B - Response of Rolling Resistance Force (Fr) Over a Range of

Three Speeds, Two Pressures, and Two Loads..................................................................... 88

Figure 54. ISO 28580 Test Conditions for Standard Load Passenger Tires................................. 89

Figure 55. Theoretical Single-Load Rolling Resistance (Fr)........................................................ 90

Figure 56. Theoretical Single-Load Rolling Resistance Coefficient (Cr) .................................... 91

Figure 57. Rolling Resistance of 16 Passenger Tires ................................................................... 92

Figure 58. Rolling Resistance Coefficient of 16 Passenger Tires ................................................ 93

Figure 59: Rolling Resistance Force (SAE J1269 Single-Point, Pounds) .................................... 97

Figure 60: Rolling Resistance Coefficient (SAE J1269) .............................................................. 97

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LIST OF TABLES Table 1. 2005 Motor Vehicle Crash Data From FARS and GES, Crashes by Weather Condition 6

Table 2. Phase 2 Tire Models ......................................................................................................... 8

Table 3. Test Matrix...................................................................................................................... 13

Table 4. Analysis of Tread Composition by TGA........................................................................ 14

Table 5. DMA Results for Tangent at 0C and 60C ................................................................ 16

Table 6. 2008 EPA Fuel Economy 5-Driving Schedule Test (Source: EPA, 2009)..................... 19

Table 7. Fuel Economy Test Schedules........................................................................................ 21

Table 8. Phase 2 Wet and Dry Skid-Trailer Test Tires................................................................. 23

Table 9. On-Vehicle Treadwear Testing....................................................................................... 24

Table 10. Indoor Treadwear Testing............................................................................................. 26

Table 11. Test Parameters............................................................................................................. 26

Table 12. Test Matrix by Date ...................................................................................................... 29

Table 13. Events Identified as Possible Data Shift Correlates...................................................... 31

Table 14. Analysis of Variance for Highway FET Fuel Economy by Tire Type and Collection

Bag Number .......................................................................................................................... 33

Table 15. Air Conditioning SC03 Schedule, mpg for SRTT Tire by Date................................... 34

Table 16. Change in Fuel Economy Over Total Time of Testing ................................................ 36

Table 17. Data Excluded from Fuel Economy Analyses.............................................................. 37

Table 18. ANOVA Results for Effect of Tire Rolling Resistance on Fuel Economy .................. 38

Table 19. Percentage Change in Fuel Economy Versus Percentage Change in Tire Rolling

Resistance ............................................................................................................................. 38

Table 20. Predicted Change in Fuel Economy for 1 psi Change in Tire Inflation Pressure......... 44

Table 21. ANOVA Results for Effect of Tire Inflation Pressure Reduction on Fuel Economy... 50

Table 22. Correlation of Rolling Resistance to Tangent at 60C .............................................. 52

Table 23. Correlation of Properties to Rolling Resistance ........................................................... 53

Table 24. Dry Traction Results, Traction Number and Ratio to E501 Reference Tire ................ 54

Table 25. Pearson Product Moment Correlation of Dry Traction to Rolling Resistance ............. 54

Table 26. Wet Traction Results, Traction Number and Ratio to E501 Reference Tire................ 57

Table 27. Pearson Product Moment Correlation of Wet Traction to Rolling Resistance............. 57

Table 28. Pearson R Product Moment Correlation of Wet Traction to ........................................ 62

Table 29. Analysis of Tire Wear Data .......................................................................................... 63

Table 30. Wear Rates and Projected Mileage to 2/32nds Tread Depth .......................................... 64

Table 31. Indoor Treadwear Tire Wear Data................................................................................ 66

Table 32. Projected Mileage to 2/32nds Inch of Tread Depth ........................................................ 66

Table 33. Projected Lifetime Versus Rolling Resistance Mild Wear at Tread Center .............. 69

Table 34. Projected Lifetime Versus Rolling Resistance Severe Wear at Tread Center ........... 70

Table 35. Projected Lifetime Versus Rolling Resistance Mild Wear at Shoulder..................... 71

Table 36. Projected Lifetime Versus Rolling Resistance Severe Wear at Shoulder ................. 72

Table 37. Analysis of Rolling Resistance Change........................................................................ 76

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LIST OF EQUATIONS Equation 1. Rolling Resistance Calculation, Force Method (ISO 28580)...................................... 9

Equation 2. Rolling Resistance Calculation, Torque Method (ISO 28580) ................................. 10

Equation 3. Input Cycle ................................................................................................................ 27

Equation 4. SAE J1269 Linear Regression Equation for Passenger Car Tires............................. 87

Equation 5. ISO 28580 Rolling Resistance Coefficient................................................................ 89

Equation 6. T&RA Load Formula for P Type Tires (S.I. Units) ............................................ 100

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DEFINITIONS

SAE The Society of Automotive Engineers International is an international standards organization providing voluntary standards to advance the state of technical and engineering sciences. SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel 877-606-7323, www.sae.org

ISO The International Organization for Standardization is a worldwide federation of national standards bodies that prepares standards through technical committees comprised of international organizations, governmental and non-governmental, in liaison with ISO. ISO Central Secretariat, 1, ch. de la Voie-Creuse, Case postale 56, CH-1211 Geneva 20, Switzerland, Telephone +41 22 749 01 11, Fax +41 22 733 34 30, www.iso.org

SAE J1269 (Rev. September 2006) SAE multi-point standard: Rolling Resistance Measurement Procedure for Passenger Car, Light Truck and Highway Truck and Bus Tires: This procedure is intended to provide a standard method for gathering data on a uniform basis, to be used for various purposes (for example, tire comparisons, determination of load or pressure effects, correlation with test results from fuel consumption tests, etc.). A single-point test condition (SRC or standard reference condition) is included. The rolling resistance at this condition may be calculated from regression of the multi-point measurements or measured directly at the SRC.

SAE J2452 (Issued June 1999) Stepwise Coastdown Methodology for Measuring Tire Rolling Resistance: This SAE Recommended Practice establishes a laboratory method for determination of tire rolling resistance of Passenger Car and Light Truck tires. The method provides a standard for collection and analysis of rolling resistance data with respect to vertical load, inflation pressure, and velocity. The primary intent is for estimation of the tire rolling resistance contribution to vehicle force applicable to SAE Vehicle Coastdown recommended practices J2263 and J2264.

ISO 18164:2005(E) Passenger car, truck, bus and motorcycle tires -- Methods of measuring rolling resistance: This International Standard specifies methods for measuring rolling resistance, under controlled laboratory conditions, for new pneumatic tyres designed primarily for use on passenger cars, trucks, buses and motorcycles.

ISO 28580 Draft International Standard (DIS) Tyre Rolling Resistance measurement method single-point test and measurement result correlation designed to facilitate international cooperation and, possibly, regulation building. Passenger Car, Truck and Bus Tyres: This recommendation specifies methods for measuring rolling resistance, under controlled laboratory conditions, for new pneumatic tyres designed primarily for use on passenger cars, trucks and buses. Tyres intended for temporary use only are not included in this specification. This includes a method for correlating measurement results to allow inter-laboratory comparisons. Measurement of tyres using this method enables comparisons to be made between the rolling resistance of new test tyres when they are free-rolling straight ahead, in a position perpendicular to the drum outer surface, and in steady-state conditions.

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http://www.sae.org/http://www.iso.org/

Rolling Resistance (Fr) (ISO/DIS 28580) Loss of energy (or energy consumed) per unit of distance travelled. NOTE 1: The SI unit conventionally used for the rolling resistance is the newton metre per metre (N m/m). This is equivalent to a drag force in newtons (N). (Also referred to as RRF).

Rolling Resistance Coefficient (Cr) (ISO/DIS 28580) Ratio of the rolling resistance, in newtons, to the load on the tyre, in knewtons. This quantity is dimensionless. (Often multiplied by 1000 kg/metric tonne (MT) for reporting. Also referred to as RRC).

Mean Equivalent Rolling Force (MERF) (SAE 2452) The average rolling resistance of a tire, at a given load/inflation condition, over a driving cycle with a specified speed-time profile. This implicitly weights the rolling resistance for each speed using the length of time spent at that speed during the cycle. For the purpose of this document, MERF is a combined weighting of MERFs calculated using the standard EPA urban and highway driving cycles. Specifically, this weighting is 55 percent for the EPA Urban (FTP) Cycle and 45 percent for the EPA Highway Fuel Economy Cycle.

Standard Mean Equivalent Rolling Force (SMERF) (SAE 2452) For any tire is the MERF for that tire under standard load/inflation conditions defined in 3.10. For this document, the final SMERF is also calculated by weighting the SMERF obtained for the EPA urban and highway cycles, as discussed previously for MERF calculation.

Tire Spindle Force, Ft (ISO/DIS 28580) Force measured at the tire spindle in newtons.

Tire Input Torque, Tp (ISO/DIS 28580) Torque measured in the input shaft at the drum axis, measured in newton-meters.

Capped Inflation (ISO/DIS 28580) Inflating the tire and fixing the amount of inflation gas in the tire. This allows the inflation pressure to build up, as the tire is warmed up while running.

Parasitic Loss (ISO/DIS 28580) Loss of energy (or energy consumed) per unit of distance excluding internal tire losses, and attributable to aerodynamic loss of the different rotating elements of the test equipment, bearing friction, and other sources of systematic loss which may be inherent in the measurement.

Skim Test Reading (ISO/DIS 28580) Type of parasitic loss measurement, in which the tire is kept rolling, without slippage, while reducing the tire load to a level at which energy loss within the tire itself is virtually zero.

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EXECUTIVE SUMMARY

The first phase of development of the tire fuel efficiency rating system consisted of the evaluation of five laboratory rolling resistance test methods, using 25 light-vehicle tire models, in duplicate at two independent laboratories. Results of this evaluation are documented in the Phase 1 report on the project. The agencys evaluation showed that all of the rolling resistance test methods had very low variability and all methods could be cross-correlated to provide the same information about individual tire types. The rank order grouping of tire types was statistically the same for each of the rolling resistance test methods evaluated. However, the relative rankings of the tires within the population of the 25 models tested shifted considerably when tires were ranked by either rolling resistance force or rolling resistance coefficient.

It was concluded from Phase 1 that while multi-point rolling resistance test methods are necessary to characterize the response of a tires rolling resistance over a range of loads, pressures, and/or speeds, either of the two shorter and less expensive single-point test methods were sufficient for the purpose of simply assessing and rating individual tires in a common system. Of the two single-point methods, the ISO 28580 Draft International Standard (DIS) has the advantage of using defined lab alignment tires to allow comparison of data between labs on a standardized basis. The use of any of the other single or multi-point test standard would require extensive development of a method to allow direct comparison of results generated in different laboratories, or even on different machines in the same laboratory. In addition, the Commission of the European Communities (EU) has selected ISO 28580 international standard as the basis of its rolling resistance rating system. Use of ISO 28580 would allow international harmonization of U.S. and European test practices.

This report summarizes the results of testing done to examine possible correlations between tire rolling resistance levels and operating parameters such as vehicle fuel economy, wet and dry traction, and outdoor and indoor treadwear. With the exception of the OE tires on the fuel economy vehicle, all tires used in Phase 2 were previously tested in one to two indoor rolling resistance tests in Phase 1. Fifteen different tire models were installed on the same new passenger car to evaluate the effects of tire rolling resistance levels on vehicle fuel economy using a test that approximately followed the EPAs new 5-cyle dynamometer test. A 10-percent decrease in tire rolling resistance resulted in approximately 1.1-percent increase in fuel economy for the vehicle. This result was within the range predicted by technical literature. Reducing the inflation pressure by 25 percent resulted in a small but statistically significant decrease of approximately 0.3 to 0.5 miles per gallon for four of the five fuel economy cycles, excluding the high-speed, high-acceleration US06 cycle. This value was smaller than many values predicted by technical literature, and possible explanations are being explored.

Sixteen tire models were subjected to dry and wet skid-trailer testing on asphalt and concrete skid pads. Both the peak (maximum) and slide (fully locked-tire) coefficients of friction were measured and indexed against the control tire. For the tires studied, there appeared to be no significant relationship between dry peak or slide numbers and rolling resistance. However, these tire models exhibited a strong and significant relationship between better rolling resistance and poorer wet slide numbers. The peak wet slide number displayed the same tendency, but the relationship was much weaker. This may be significant to consumers without anti-lock braking sys

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tems (ABS) on their vehicles since the wet slide value relates most closely to locked-wheel emergency stops. For newer vehicles with ABS or electronic stability control systems, which operate in the earlier and higher peak friction range, the tradeoff is less significant. The agencys current Uniform Tire Quality Grading Standards (UTQGS) (575.104) rate wet slide traction but not wet peak traction. For the subset of five tire models subjected to on-vehicle treadwear testing (UTQGS), no clear relationship was exhibited between tread wear rate and rolling resistance levels. For the subset of six tire models subjected to significant amounts of wear in the indoor treadwear tests, there was a trend toward faster wear for tires with lower rolling resistance.

The Requirements section of the report contains an analysis of the various options in the draft ISO 28580 rolling resistance test and their likelihood of inducing variability in the test results. The lab alignment procedure in ISO 28580, which for passenger tires uses two dissimilar tires to calibrate a test lab to a master lab, states that it will compensate for differences induced from tests conducted using different options under the test standard. These options include the use of one of four measurement methods (force, torque, power, or deceleration), textured or smooth drum surface, correction of data to a 25C reference temperature, and correction of data from tests conducted on a test drum of less than 2.0-m in diameter to a 2.0-m test drum. The variability in test results induced by allowing the various test options, as well as the effectiveness of the temperature and test drum correction equations is not currently known to the agency. Some recommendations are included.

Concluding the report is a special discussion regarding the use of rolling resistance (Fr) or rolling resistance coefficient (Cr) as the basis for data reporting and ratings. The ISO 28580 standard calculates a rolling resistance (Fr, energy loss per unit distance) from one of four different measurement methods. Since, rolling resistance varies with the load on the tire, and tires of different load indexes are tested at different loads, the rolling resistance coefficient is used to allow a relative comparison of the energy consumption of tires of all sizes and load ranges. However, the normalization of Fr to generate Cr is not consistent across the range of tire sizes and load ranges in what is expected to be about 20,000 different tires in a common system. If the Cr coefficient is used as a basis, the data will be skewed towards better ratings for larger tires. While this would have negligible effects for consumers picking out tires of a given size, there are concerns about the confusion of consumers if the overall tire fuel economy system was to rate tires that consume more fuel at a given set of conditions better than tires that consume less fuel at those same conditions.

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1.0 INTRODUCTION

Reducing energy consumption is a national goal for many reasons, from economic and national security to improving air quality and reducing greenhouse gas emissions. Also, rising energy prices are having their effect on consumers and businesses, and have contributed to increases in the Consumer Price Index in recent years. Hall and Moreland define tire rolling resistance as the energy consumed per unit distance of travel as a tire rolls under load.[1] A vehicles fuel economy is affected by tire rolling resistance, therefore, fuel saving could be achieved by reducing tire rolling resistance. Low-rolling-resistance original equipment (OE) tires are used by auto manufactures to help meet the Federal fuel economy standards for new passenger cars and light trucks. However, consumers often purchase less fuel-efficient tires when replacing their vehicles OE tires, as well as when purchasing subsequent sets of replacement tires. For example, during 2007 there were an estimated 51 million OE passenger and light truck tires sold in the United States, as opposed to an estimated 237 million replacement passenger and light truck tires.[2] Therefore, the rolling resistance of replacement tires could have a significant impact on the fuel economy of the U.S. light-vehicle fleet.

In the Consolidated Appropriations Act of 2004, Congress provided funding through the NHTSA to the National Academy of Sciences (NAS)1 to develop and perform a national tire fuel efficiency study and literature review.[3] The NAS was to consider the relationship that low rolling resistance tires designed for use on passenger cars and light trucks have with vehicle fuel consumption and tire wear life. The study was to address the potential of securing technically feasible and cost-effective fuel savings from low rolling resistance replacement tires that do not adversely affect tire safety, including the impacts on performance and durability, or adversely impact tire tread life and scrap tire disposal, and that does fully consider the average American drive cycle. The study was to further address the cost to the consumer including the additional cost of replacement tires and any potential fuel savings. The resulting NAS Transportation Research Board report of April 2006 concluded that reduction of average rolling resistance of replacement tires by 10 percent was technically and economically feasible, and that such a reduction would increase the fuel economy of passenger vehicles by 1 to 2 percent, saving about 1 to 2 billion gallons of fuel per year nationwide. However, as is common in such studies, the NAS committee did not have a mechanism to generate its own test data2 and conclusions were based upon available literature and data.[4] The tire industry eventually supplied rolling resistance data for 214 passenger and light truck tire models to the NAS committee (177 Michelin

1 Ultimately the Committee for the National Tire Efficiency Study of the Transportation Research Board, a division of the National Research Council that is jointly administered by the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. 2 NAS cautioned that much of the available technical literature on tire rolling resistance dates back to the mid-1970s to mid-1980s. Data on todays passenger tires was difficult to obtain.

1

manufactured, 24 Bridgestone-manufactured, and 13 Goodyear-manufactured passenger and light truck tires).3

The Transportation Research Board report suggests that safety consequences of a 10-percent improvement in tire rolling resistance were probably undetectable. However, the committees analysis of grades under UTQGS (FMVSS No. 575.104) for tires in its study indicated that there was difficulty in achieving the highest wet traction and/or treadwear grades while achieving the lowest rolling resistance coefficients. This was more noticeable when the sample of tires was constrained to similar designs (similar speed ratings and diameters). A lack of access to the raw rating numbers instead of the final grades provided by the manufacturers prohibited a more detailed analysis.

Subsequent to the publication of the NAS committee report, NHTSA initiated a research program to evaluate five laboratory rolling resistance test methods, using 25 currently available light vehicle tire models, in duplicate at two independent laboratories. Results of this evaluation are documented in the Phase 1 report of the project. The agencys evaluation showed that all of the rolling resistance test methods had very low variability and all methods could be cross-correlated to provide the same information about individual tire types. Differences of as much as 30 percent in measured rolling resistance force were observed between different models of tires of the same size. It was concluded that while multi-point rolling resistance test methods are necessary to characterize the response of a tires rolling resistance over a range of loads, pressures, and/or speeds, either of the two shorter and less expensive single-point test methods were sufficient for the purpose of simply assessing and rating individual tires in a common system. Of the two single-point methods evaluated, the ISO 28580 Draft International Standard (DIS) has the advantage of using defined lab alignment tires to allow comparison of data between labs on a standardized basis. The use of any of the other single or multi-point test standard would require extensive development of a method to allow direct comparison of results generated in different laboratories, or even on different machines in the same laboratory. Also, the Commission of the European Communities (EU) has selected ISO 28580 international standard as the basis of its rolling resistance rating system. Use of ISO 28580 would allow international harmonization of U.S. and European test practices.

In December 2007, Congress enacted the Energy Independence and Security Act of 2007 that mandated that NHTSA establish a national tire fuel efficiency rating system for motor vehicle replacement tires within 24 months. While the existing research program was sufficient to meet the requirements for the testing and rating requirements, NHTSA initiated a second phase of research to address the safety and consumer information requirements. Portions of Phase 2 of the project retested up to 15 models of Phase 1 tires, as well the original equipment tires on the fuel economy test vehicle, to examine possible correlations between tire rolling resistance levels and operating parameters such as vehicle fuel economy, wet and dry traction, and outdoor and indoor

3 NAS: Before the committees final meeting, several tire manufacturers, acting through the Rubber Manufacturers Association, made available measurements of the rolling resistance of a sample of more than 150 new replacement passenger tires as well as some original equipment (OE) tires. Although the sample was not scientifically derived, the data proved helpful to the committee as it sought to answer the various questions in the study charge. The timing of the datas availability late in the study process limited the statistical analyses that could be undertaken by the committee. Reference [4], Page ix.

2

treadwear. This was accomplished through on-vehicle EPA dynamometer fuel economy tests, wet and dry skid-trailer traction tests, on-vehicle treadwear tests and experimental indoor tread-wear tests.

1.1 The Concept of Rolling Resistance

In the latest version of the book The Pneumatic Tire, which was commissioned and published by NHTSA, LaClair describes the concept of rolling resistance in simple terms[5]:

When a tire rolls on the road, mechanical energy is converted to heat as a result of the phenomenon referred to as rolling resistance. Effectively, the tire consumes a portion of the power transmitted to the wheels, thus leaving less energy available for moving the vehicle forward. Rolling resistance therefore plays an important part in increasing vehicle fuel consumption. Rolling resistance includes mechanical energy losses due to aerodynamic drag associated with rolling, friction between the tire and road and between the tire and rim, and energy losses taking place within the structure of the tire.

LaClair also points out that the term rolling resistance is often mistaken as a measure of the force opposing tire rotation, when instead is actually a measure of rolling energy loss[6]:

Although several authors recognized the importance of energy consumption, the concept of rolling resistance as a retarding force has persisted for many years. Schuring provided the following definition of rolling resistance as a loss in mechanical energy: Rolling [resistance] is the mechanical energy converted into heat by a tire moving for a unit distance on the roadway. He proposed the term rolling loss instead of rolling resistance so that the long-standing idea of a force would be avoided. Schuring pointed out that although rolling resistance -- defined as energy per unit distance -- has the same units as force (J/m = N), it is a scalar quantity with no direction associated with it.

Defining rolling resistance as an energy loss is advantageous when considering its effects on the fuel efficiency of a vehicle. The U.S. Department of Energy estimates that approximately 4.2 percent of the total energy available in the fuel you put in your tank is lost to rolling resistance during the operation of the vehicle (Figure 1).[7] However, Duleep and NAS point out that the peak first law (thermodynamic) efficiency of a modern spark-ignited gasoline engine is in the 3436 percent range (40-42% for diesels), and therefore tire rolling resistance consumes about a third of the usable energy actually transmitted to the wheels (i.e., 1/3 of the available tractive energy). Therefore, considering rolling resistance in terms of the energy in the fuel tank is not a useful measure.[8],[9] For instance, in Figure 1 only 12.6 percent of the energy in the fuel is finally transmitted to the wheels. The 4.2 percent of original fuel energy used by rolling resistance is actually 33 percent (4.2%/12.6%) of the total usable energy available to the wheels.

3

Only about 15 percent of the energy from the fuel you put in your tank gets used to move your car down the road or run useful accessories, such as air conditioning. The rest of the energy is lost to engine and driveline inefficiencies and idling. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous.

Rolling Resistance 4.2 percent For passenger cars, a 5 to 7 percent reduction in rolling resistance increases fuel efficiency by 1 percent. However, these improvements must be balanced against traction, durability, and noise.

Figure from Department of Energy, 2009 Figure 1. Where Does the Energy Go?

Additionally, the contribution of tire rolling resistance to fuel economy varies with the speed of the vehicle. At lower speeds, tire rolling resistance represents a larger percentage of the fuel consumption (Figure 2) than at higher speeds.[10]

4

Figure 2. Contribution of Tire Rolling Resistance to Vehicle Fuel Economy Versus Speed (Reprinted with permission from the Automotive Chassis: Engineering Principles,

2nd Edition, Reed Educational and Professional Publishing Ltd., 2001)

In any discussion of rolling resistance, it is important to consider that the rolling resistance level of a tire evolves during use. It is reported in literature that a tires rolling resistance level, and therefore its effects on vehicle fuel economy, can decrease by more than 20 percent from a new tread to completely worn.[11],[12] Therefore, calculations of the benefits of lower tire rolling resistance derived from measurements of new tires will likely understate the benefits to a vehicle in terms of absolute fuel economy over the lifetime of the set of tires. However, since both new-vehicle fuel economy and new-tire rolling resistance change with time, and are dependent on usage conditions, age, and maintenance levels, attempts to calculate lifetime benefit can vary widely.

While the hysteretic losses of the tire (primarily the tread) consume a large amount of the available tractive energy, the tires also provide the traction necessary to start, stop, and steer the vehicles. Substances soft enough to provide traction on wet, dry, snow, dirt, gravel, etc., surfaces will also wear. Therefore, the topics of rolling resistance, traction, and treadwear are linked in what the tire industry refers to as the magic triangle (Figure 3). The triangle is a useful graphic since it conveys the point that a shift to improve properties in one corner of the triangle can diminish properties in both of the other corners if more advanced and often more expensive tire compounding and construction technologies are not employed.

5

Rolling Resistance

Traction

Treadwear Figure 3. Magic Triangle: Traction, Treadwear, and Rolling Resistance

From a safety standpoint, the obvious concern from the magic triangle is a loss of tire traction to achieve lower rolling resistance (better vehicle fuel economy). Since 85 percent of all crashes in 2005 occurred during normal dry weather conditions, and 10 percent in the rain (Table 1.), the effects of lower rolling resistance on wet and dry traction are of primary importance.[13] Longitudinal wet and dry tire traction are easily measured with skid-trailer testing. Conversely, while crashes occur on snow, sleet, and ice about 4 percent of the time, measuring tire traction on the varying permutations of these surfaces is not easily done.

Table 1. 2005 Motor Vehicle Crash Data From FARS and GES, Crashes by Weather

Condition

Weather Condition All Crashes Percent Normal (dry) 5,239,000 85.1% Rain 584,000 9.5% Snow/Sleet 264,000 4.3% Other 72,000 1.2% Total 6,159,000 100%

6

2.0 METHODOLOGY

2.1 Test Tires

The majority of the tire models selected for Phase 1 were size P225/60R16 or 225/60R16, which in 2007 was the most popular size of replacement tire in the United States. Phase 1 of the project evaluated the rolling resistance of 25 passenger and light-truck tire models. However, time and budget constraints, as well as equipment limitations, limited Phase 2 to retests of 5 to 16 of the Phase 1 models in different portions of the project (Table 2). The original equipment tires on the fuel economy test vehicle added a 17th tire model to the Phase 2 test matrix. The Phase 2 tire models ranged from 14- to 17-inch rim codes, Q to W speed ratings, 9 to 15 lbf (7 to 11 Cr) in rolling resistance per ISO 28580, 19 to 36 lbs in weight, 300 to 700 in treadwear rating, and A to AA in UTQGS traction (wet) rating.

The Phase 1 passenger tires, all purchased as new, were not subjected to optional break-ins listed in the various rolling resistance tests prior to the warm-up and measurement phases of the tests. Therefore, Phase 1 tires experienced approximately 50 to 75 miles of straight-line mileage on the laboratory rolling resistance machine prior to Phase 2 testing. This produced no detectable treadwear, but did serve to break-in the tires. It has been reported by LaClair that tire rolling resistance will decrease about 2-5 percent during a break-in period of 60 minutes at 80 km/h (50 total miles).[14] Therefore, it is anticipated that the rolling resistance of the tires retested in Phase 2 for on-vehicle fuel economy, traction, and treadwear is approximately 2-5 percent lower than a brand new tire subjected to these tests. However, it should also be noted that most of these tests are normally completed with tires that are broken-in prior to testing (vehicle fuel economy - 2,000 miles, outdoor traction - 200 miles, outdoor treadwear - 800 miles).

2.1.1 ASTM F2493 Radial Standard Reference Test Tire

Tire model M14 is an ASTM F2493 SRTT tire. The ASTM F2493 - Standard Specification for P225/60R16 97S Radial Standard Reference Test Tire (SRTT) provides specifications for a tire for use as a reference tire for braking traction, snow traction, and wear performance evaluations, but may also be used for other evaluations, such as pavement roughness, noise, or other tests that require a reference tire. The standard contains detailed specifications for the design, allowable dimensions, and storage of the tires. The F2493 SRTT is a variant of a modern 16-inch Uniroyal TigerPaw radial passenger vehicle tire and comes marked with a full USDOT Tire Identification Number and UTQGS grades. The SRTTs were used extensively throughout the laboratory, test surface, and fuel economy phases of the test program to monitor the stability of the testing. The SRTTs had the added advantage of being near the center of the range of passenger tire rolling resistances in the program (Table 2).

7

Table 2. Phase 2 Tire Models Ti

re M

odel

Cod

e

MFG

Size

Load

Inde

x

Spee

d R

atin

g

Mod

el

UTQ

GS

Trea

d-w

ear

UTQ

GS

Trac

.

UTQ

GS

Tem

p.

Perf

orm

ance

Le

vel

ISO

285

80 R

ollin

g R

esis

tanc

e, F

r (lb

f)

ISO

285

80 R

ollin

gR

esis

tanc

e C

oef

ficie

nt ,

Cr

Wei

ght (

lbs.

)

G12 Goodyear P225/60R16 97 S Integrity 460 A B Passenger All Season, TPC 1298MS

9.47 7.36 22.0

G8 Goodyear 225/60R16 98 S Integrity 460 A B Passenger All Season

9.83 7.44 22.9

G11 Goodyear P225/60R17 98 S Integrity 460 A B Passenger All Season

10.02 7.58 24.5

B11 Bridgestone P225/60R16 97 H Potenza RE92 OWL

340 A A High Performance All Season

10.13 7.87 25.1

G9 Goodyear P205/75R14 95 S Integrity 460 A B Passenger All Season

11.27 9.19 19.2

M14 Uniroyal P225/60R16 97 S ASTM 16" SRTT

540 A B ASTM F 2493-06 Reference

11.96 9.30 25.5

M13 Michelin 225/60R16 98 H Pilot MXM4 300 A A Grand Touring All Season

12.07 9.13 24.7

G10 Goodyear P205/75R15 97 S Integrity 460 A B Passenger All Season

12.09 9.46 20.4

B10 Bridgestone 225/60R16 98 Q Blizzak REVO1*

- Performance Winter 12.11 9.16 26.9

D10 Cooper 225/60R16 98 H Lifeliner Touring SLE

420 A A Standard Touring All Season

13.56 10.26 25.2

B14 Bridgestone P225/60R16 97 V Turanza LS-V 400 AA A Grand Touring All Season

13.90 10.80 28.6

U3 Dunlop (Sumitomo)

P225/60R17 98 T SP Sport 4000 DSST

360 A B Run Flat 13.91 10.52 36.4

B15 Dayton 225/60R16 98 S Winterforce* - Performance Winter 13.99 10.58 26.7

P5 Pep Boys (Cooper)

P225/60R16 97 H Touring HR 420 A A Passenger All Season

14.02 10.89 25.7

R4 Pirelli 225/60R16 98 H P6 Four Seasons

400 A A Passenger All Season

14.98 11.33 24.3

B13 Bridgestone P225/60R16 97 T Turanza LS-T 700 A B Standard Touring All Season

15.01 11.66 29.4

B12 Bridgestone P225/60R16 98 W Potenza RE750 340 AA A Ultra High Performance Summer

15.22 11.51 27.4

Original equipment tires on the fuel economy test vehicle.

Standard reference test tires used as control tires throughout all phases of the study.

*Snow tires will not be rated in the national tire fuel efficiency consumer information program.

2.2 Tire Rolling Resistance Test Procedures

Tire rolling resistance is measured in a laboratory under controlled conditions. The test conditions vary between the various SAE and ISO test standards, but the basic premise is the same in that a tire is mounted on a free-rolling spindle with no camber or slip angle, loaded against a large-diameter powered test drum, turned by the drum to simulate on-road rolling operation, and some measure of rolling loss evaluated. Referring back to the book The Pneumatic Tire[5]:

Rolling resistance is the effort required to keep a given tire rolling. Its magnitude depends on the tire used, the nature of the surface on which it rolls, and the operating conditions - inflation pressure, load and speed.

8

This description is important because it emphasizes that rolling resistance is not an intrinsic property of the tire, rather a function of many operating variables. This is why multi-point laboratory tests measure a tires rolling resistance over a range of inflation pressures, loads, and for some tests, a range of speeds. Conversely, single-point point rolling resistance test methods use a single set of these variables to estimate the rolling resistance of the tire under nominal, straight-line, steady state operating conditions (the vast majority of a tires rolling operation). In the case of a laboratory test, rolling resistance (energy loss) is calculated by measuring the amount of additional force, torque, or power necessary to keep the tire rolling at the test conditions. A fourth method, which is not widely used, is a deceleration method in which the energy source is de-coupled from the system and the rate of loss of angular momentum (energy loss) imparted by the tire is measured.

The two domestic test labs used by the agency had machines that used either the force or the torque measurement method. A picture of a laboratory rolling resistance test using a force method can be seen in Figure 4. The machine measures a reaction force at the axle of the test tire & wheel assembly. The drum is brought up to speed and the tire is warmed up to an equilibrium temperature. The tire is then lightly loaded to measure parasitic losses caused by the tire spindle friction, aerodynamic losses, and the test drum/drive system bearings. The tire is then loaded to the test load and successive readings are taken until consistent force values are obtained. During the test, the loaded radius (rL) of the tire is measured during the steady-state conditions. In ISO 28580 the Rolling Resistance (Fr) at the tire/drum interface is calculated from the measured force at the spindle (Ft), multiplied by a ratio of the loaded tire radius (rL) to the test wheel radius (R), minus the skim load (Fpl).

Fr = Ft[1+(rL/R)]-Fpl

Equation 1. Rolling Resistance Calculation, Force Method (ISO 28580)

9

Ft = Spindle Force

rL

R

Fr = Calculated Rolling Resistance at Tire/Drum Interface

1.7 meter Drum

Motor

Torque Cell 1.7 meter roadwheel

80 grit Surface

T = torque

Figure 4. Force Method Rolling Resistance Test Machine

Another test lab used by the agency used a torque method machine. The torque method measures the torque required to maintain the rotation of the drum. The drum is connected to the motor through a torque cell (Figure 5). The drum is brought up to speed and the tire is warmed up to an equilibrium temperature. The tire is then lightly loaded to measure the losses caused by the axle holding the tire and aerodynamic losses from the tire spinning. The tire is then loaded to the test load and successive readings are taken until consistent torque (Tt) values are obtained.

Fr = Tt/R-Fpl

Equation 2. Rolling Resistance Calculation, Torque Method (ISO 28580)

Figure 5. Torque Method Rolling Resistance Test Machine

10

In one additional calculation, the rolling resistance force (Fr) calculated by any of the methods is divided by the nominal test load on the tire to produce the rolling resistance coefficient (Cr). Since the rolling resistance coefficient (Cr) is not linear between tires of different load ranges, the rolling resistance (Fr) for each tire was compared to the traction, treadwear, and fuel economy measures in the Phase 2 analysis.

Tires in Phases 1 and 2 were subjected to up to three tests. The first and possibly second test may have been the same indoor rolling resistance test or two different tests, followed by traction, treadwear or fuel economy testing. A detailed test matrix is provided in Appendix 2. A description of the laboratory rolling resistance tests used in Phase 1 follows:

2.2.1 ISO Draft International Standard 28580 Single-Point Rolling Resistance

Tires from all 17 tire models used in Phase 2, though not necessarily the exact tires, were previously tested using the draft ISO 28580 test method.

2.2.2 SAE J1269 & ISO 18164 Multi-Point Rolling Resistance

Tires from all 17 tire models in Phase 2, though not necessarily the exact tires, were previously tested with SAE J1269, and 11 models were previously tested with ISO 18164 (both tests are very similar). Data from this multi-point test allows estimation of tire rolling resistance at the test vehicle load and the two inflation pressures used in the vehicle fuel economy testing.

2.2.3 SAE J2452 Multi-Point (Speed Coast Down) Rolling Resistance

With the exception of the original equipment (OE) tires, tires from 16 tire models in Phase 2, though not necessarily the exact tires, were previously tested with SAE J2452. Data from this multi-point test allows estimation of tire rolling resistance at the test vehicle load, two inflation pressures, and speeds used in the vehicle fuel economy testing.

2.3 Fuel Economy Test Vehicle

A 2008 Chevrolet Impala LS was selected as the test vehicle for fuel economy testing since it came equipped with P225/60R16 tires, and GM original equipment tires have a Tire Performance Code (TPC) that allows purchase of replacement tires with the same specifications as the OE tires. These OE tires (tire type G12) became the 17th group of tires in Phase 2 and had the lowest rolling resistance of any tire tested in the program (Table 2).

2.4 Test Wheels

Tires were tested on wheels of the corresponding measuring rim width for their size. Wheels of each size used in the test program were purchased new, in identical lots to minimize wheel-towheel variation. A tire participating in multiple tests throughout the test program was mounted

11

once on a single new wheel and continued to be tested on that same wheel until completion of all tests.

2.5 Test Matrix

The EISA legislation requires a national tire fuel efficiency consumer information program to educate consumers about the effect of tires on automobile fuel efficiency, safety, and durabil-ity.[15] Phase 2 of the project was therefore designed to examine the effects of tire rolling resistance levels on vehicle fuel economy, traction, and treadwear. Phase 1 tires were retested in one of five Phase 2 test protocols: On-vehicle EPA dynamometer fuel economy (Dyno. FE), wet and dry skid-trailer traction, on-vehicle treadwear, an experimental indoor treadwear test, or tread rubber analysis by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) (Table 3). Due to time and cost considerations, as well as the physical constraints the fuel economy test vehicle and skid-trailer, the four tests used a subset of the 17 available Phase 2 tire models selected to cover the range of rolling resistance values in the experiment.

12

Table 3. Test Matrix Code MFG Size Load

Index Speed Rating

Model RR (lbf)

Dyno. FE

Wet & Dry

Traction

On-vehicle

Treadwear

Indoor Treadwear

TGA /

DMA G12 Goodyear P225/60R16 97 S Integrity 9.47 x x G8 Goodyear 225/60R16 98 S Integrity 9.83 x x x x x G11 Goodyear P225/60R17 98 S Integrity 10.02 x x x B11 Bridgestone P225/60R16 97 H Potenza

RE92 OWL 10.13 x x x x x

G9 Goodyear P205/75R14 95 S Integrity 11.27 x x M14 Uniroyal P225/60R16 97 S ASTM 16"

SRTT 11.96 x x x x x

M13 Michelin 225/60R16 98 H Pilot MXM4 12.07 x x x x x G10 Goodyear P205/75R15 97 S Integrity 12.09 x x B10 Bridgestone 225/60R16 98 Q Blizzak

REVO1 12.11 x x x

D10 Cooper 225/60R16 98 H Lifeliner Touring SLE

13.56 x x x

B14 Bridgestone P225/60R16 97 V Turanza LS-V

13.90 x x x

U3 Dunlop (Sumitomo)

P225/60R17 98 T SP Sport 4000 DSST

13.91 x x x

B15 Dayton 225/60R16 98 S Winterforce 13.99 x x x P5 Pep Boys

(Cooper) P225/60R16 97 H Touring HR 14.02 x x x

R4 Pirelli 225/60R16 98 H P6 Four Seasons

14.98 x x x

B13 Bridgestone P225/60R16 97 T Turanza LS-T

15.01 x x x x x

B12 Bridgestone P225/60R16 98 W Potenza RE750

15.22 x x x

Original equipment tires on the fuel economy test vehicle. Standard reference test tires used as control tires throughout all phases of the study.

2.6 Tread Compound Properties Testing

The tread rubber of 16 Phase 1 passenger tires was analyzed for compound composition by thermogravimetric analysis (TGA). The mechanical properties of the treads were evaluated by dynamic mechanical analysis (DMA). TGA is a useful tool for characterizing polymer compositions. The weight loss as a function of temperature has been used to determine polymer loading, rubber chemical loading, carbon black loading, and ash levels. For polymers with very different thermal stabilities, the TGA curves can be used to determine the amount of each polymer present. Thermogravimetric analysis was performed using about 10 mg of sample of each tire tread. The purge (He) gas flow rate to the TGA was set at 10ml/min during weight loss measurements. The heating rate was 10C/min to improve the resolution of small variations in the decomposition curves. At 600C, the purge gas was switched over to air for carbon black combustion. These average values represent the average of three measurements. Figure 6 shows a representa

13

Wei

ght R

etai

ned

(%)

120

100 Volatile Components

80

60 Polymer

40

20 Carbon Black

Ash (Zinc Oxide, Silica, 0

0 200 400 600 800 1000 Temperataure (degC)

tive weight loss curve with the regions that represent each component identified. The results of the TGA analysis are shown in Table 4.

Figure 6. Sample TGA Weight Loss Curve

Table 4. Analysis of Tread Composition by TGA Tire Black, Type

Tire #

Polymer,% (325-550C)

Volatiles, phr (25325C)

phr (550

850C) Ash, phr (Residue)

Total Filler, phr

Silica, phr

Total Formulation,

phr B10 3104 57 18 32 25 51 19 169 B11 3129 56.8 18 31 27 52 21 170 B12 3154 49 25 54 25 73 19 198 B13 3179 51.3 22 44 29 67 23 189 B14 3204 52 25 13 54 62 48 186 D10 3313 46.9 33 77 3 77 0 207 B15 3337 54.3 19 63 3 63 0 178 U3 3362 52.4 18 33 40 67 34 185 G8 3412 60.4 15 38 12 45 6 159 G9 3441 52.9 23 60 6 60 0 183 G10 3466 58.3 22 45 4 45 0 165 G11 3491 63.3 15 33 11 37 5 152 M13 3620 54.3 19 10 55 59 49 178

14

Tire Type

Tire #

Polymer,% (325-550C)

Black, phr Volatiles, Total Total

(550-phr (25- Ash, phr 325C) 850C) (Residue)

Filler, Silica, Formulation, phr phr phr

P5 3670 47.1 29 79 4 79 0 206 R4 3695 48.3 30 42 35 71 29 201 M14 3720 55 19 30 32 57 26 176

Typical examples of temperature sweep data by the tension method and the shear method are shown below in Figure 7 and Figure 8. The viscoelastic (dynamic mechanical) properties of a tire tread have been correlated to the performance of tires.[16],[17],[18],[19] Decreased tangent at 60C is used as a predictor of the tread compounds contribution to tire rolling resistance. In-creased tangent at 0C has been shown to correlate to the wet traction performance of the tire. Since these properties tend to move in parallel, lowering the tangent at 60C while maintaining a high tangent at 0C normally requires utilization of advanced and often more expensive com-pounding technologies. The DMA results for high tangent at 0C and 60C are shown in Table 5.

0.00.10.20.30.40.50.60.70.8

-150 -100 -50 0 50 100Temperatue (C)

Tang

ent D

elta

Figure 7. Tan as a Function of Temperature From the Tension Test

15

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

-100 -50 0 50 100

Temperature (deg C)

Tang

ent D

elta

Figure 8. Tan as a Function of Temperature From the Shear Test

Table 5. DMA Results for Tangent at 0C and 60C

Tire Type

Tire #

Rolling Resistance*

(lbf)

Tension Shear Tan at

0C Tan at

60C Ratio 0/60 Tan at

0C Tan at

60C Ratio 0/60

G8 3412 9.83 0.169 0.0762 2.22 0.164 0.0689 2.38 G11 3491 10.02 0.174 0.086 2.02 0.177 0.0754 2.35 B11 3129 10.13 0.194 0.0771 2.52 0.174 0.067 2.60 G9 3441 11.26 0.245 0.188 1.30 0.18 0.152 1.18 M14 3720 11.96 0.287 0.193 1.49 0.202 0.146 1.38 M13 3620 12.06 0.254 0.147 1.73 0.168 0.117 1.44 G10 3466 12.09 0.242 0.181 1.34 0.184 0.151 1.22 B10 3104 12.11 0.2 0.155 1.29 0.16 0.133 1.20 D10 3313 13.56 0.26 0.192 1.35 0.183 0.16 1.14 B14 3204 13.90 0.313 0.145 2.16 0.233 0.132 1.77 U3 3362 13.91 0.256 0.173 1.48 0.202 0.147 1.37 B15 3337 13.98 0.208 0.15 1.39 0.158 0.123 1.28 P5 3670 14.02 0.271 0.207 1.31 0.161 0.156 1.03 R4 3695 14.98 0.296 0.201 1.47 0.211 0.159 1.33 B13 3179 15.01 0.265 0.168 1.58 0.19 0.138 1.38 B12 3154 15.22 0.387 0.193 2.01 0.28 0.146 1.92 *ISO 28580 single-point rolling resistance

2.7 On-Vehicle Fuel Economy Testing

The effects of tire rolling resistance on automobile fuel efficiency was evaluated by installing 15

different tire models on a new 2008 Chevrolet Impala LS and evaluating its fuel economy in the

2008 five-cycle EPA fuel economy test.[20] Testing was completed under contract by the Transportation Research Center, Inc. (TRC, Inc.) emissions laboratory. Since tire inflation pressure

affects the operational rolling resistance of a tire, the vehicle fuel economy measurements were

conducted at two different tire inflation pressures. Testing was completed at the vehicle placard

16

pressure of 210 kPa (30 psi). Six models were tested at both the placard inflation pressure of 210 kPa and at 158 kPa (23 psi), which represents the tire pressure monitoring system (TPMS) activation threshold of 25 percent inflation pressure reduction. It is important to note, for reasons that will be explained, that these tests were research and not official EPA fuel economy ratings of the test vehicle. The many tire sets and repeats of test for statistical analysis/dual inflation pressure resulted in the test vehicle acquiring nearly 6,000 miles by the end of testing. The EPA estimates that new vehicles will not obtain their optimal fuel economy until the engine has broken in at around 3,000 to 5,000 miles.[21] Therefore the fuel economy of the test vehicle was expected to improve slightly during the course of testing, a factor that was tracked and accounted for by the repeated testing of the control and OE tires at regular intervals throughout the testing.

2.7.1 EPA 40 CFR Part 86 Dynamometer Fuel Economy Testing

Per EPA 40 CFR Part 86, the new 2008 Chevrolet Impala LS test vehicle was broken in for 2,000 miles on a test track. To keep the original equipment tires in the same low mileage state as the Phase 1 tires, the vehicle was broken-in on a spare set of replacement tires of the original equipment size. For this reason, even the fuel economy tests of the Impala with the original equipment tires were not official EPA test numbers. The original equipment tires were reinstalled on the vehicle at placard inflation pressure and the road load coastdown procedure was completed. The coastdown procedure generates vehicle-specific coefficients for dynamometer settings and fuel economy calculations.

The fuel economy dynamometer is housed in an environmental chamber to control the temperature for ambient (68 to 86 degrees F), heated (95 degrees F) or cold (20 degrees F) temperatures. The vehicle dynamometer is a 1.22-meter (48-inch) diameter, smooth surface drum located in the floor of the chamber. The vehicle is placed atop the dynamometer rolls and restrained to prevent movement (Figure 9a). A fan meeting standard specifications is located in front of the vehicle to provide cooling (Figure 9b). A computer is mounted inside the vehicle to provide the driver with a prescribed speed pattern that must be followed for each test cycle (Figure 9c). The exhaust gas is routed from the vehicle exhaust tailpipe via hoses to a collection system connected to gas analyzers (Figure 9d).

17

Figure 9a. Tire on 1.22 Meter Dynamometer Figure 9b. Chamber and Fan

Figure 9c. Drive Cycle Computer Figure 9d. Exhaust Coupling Figure 9. Vehicle Fuel Economy Dynamometer Testing

Details of the 2008 EPA fuel economy test can be found in Table 6, which is from the EPAs www.fueleconomy.gov Website.[22]

18

http://www.fueleconomy.gov/

Table 6. 2008 EPA Fuel Economy 5-Driving Schedule Test (Source: EPA, 2009) Driving Schedule Attributes

Test Schedule

City (FTP) Highway (HwFET)

High Speed (US06)

AC (SC03) Cold Temp (Cold CO)

Trip Type Low speeds in stop-and-go urban traffic

Free-flow traffic at highway speeds

Higher speeds; harder acceleration & braking

AC use under hot ambient conditions

City test w/ colder outside temperature

Top Speed 56 mph 60 mph 80 mph 54.8 mph 56 mph Average Speed

21.2 mph 48.3 mph 48.4 mph 21.2 mph 21.2 mph

Max. Acceleration

3.3 mph/sec 3.2 mph/sec 8.46 mph/sec 5.1 mph/sec 3.3 mph/sec

Simulated Distance

11 mi. 10.3 mi. 8 mi. 3.6 mi. 11 mi.

Time 31.2 min. 12.75 min. 9.9 min. 9.9 min. 31.2 min. Stops 23 None 4 5 23 Idling time 18% of time None 7% of time 19% of time 18% of time Engine Startup*

Cold Warm Warm Warm Cold

Lab temperature

68-86F 95F 20F

Vehicle air conditioning

Off Off Off On Off

*A vehicle's engine doesn't reach maximum fuel efficiency until it is warm.

Whole vehicle preconditioning must be done between the ambient and cold test cycles. Therefore, instead of running all five fuel economy cycles sequentially in their traditional order, testing with the 15 sets of tires was split into blocks that facilitated a much more rapid test throughput. In addition, to gather more data for statistical purposes, two extra HwFET cycles were run sequentially after the first HwFET cycle. The testing was conducted at the placard tire inflation pressure of 210 kPa (30 psi) and repeated at the TPMS warning activation pressure of 158 kPa (22.3 psi) for selected tires.

Vehicle Preconditioning Vehicle preconditioning begins with draining the existing fuel from the vehicles fuel tank and replacing it with a 40 percent fuel tank capacity fill of the specified fuel. The vehicle is then driven through one Urban Dynamometer Driving Schedule (UDDS). This procedure is followed by a soak period of at least 12 hours, but not exceeding 36 hours. All preconditioning procedures are performed at the conditions of the test schedule.

FTP Schedule Testing Following the vehicles soak period, the vehicle is pushed, not driven, onto a chassis dynamometer for a cold start exhaust emissions test (75 FTP). The Federal test procedure (FTP) simulates normal city driving and collects dilute exhaust emissions into bags for analysis in three phases: the cold transient (CT), the cold stable (CS), and the hot transient (HT). The UDDS is followed during the CT and CS, and, following a ten-minute soak on the dynamometer, the first phase, or

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bag, of the UDDS is repeated for the HT. The results of these phases are combined to provide grams per mile (g/mi) for total hydrocarbons (THC), non-methane hydrocarbons (NMHC), carbon monoxide (CO), carbon dioxide (CO2) and oxides of nitrogen (NOx). Fuel economy, in miles per gallon, is determined via the carbon balance method.

HwFET Schedule Testing Following each FTP test, the vehicle is kept on the chassis dynamometer and the Highway FET (HwFET) driving cycle was run twice. The first running of the HwFET served only to stabilize vehicle temperatures and emissions, therefore fuel economy was not measured during this cycle. The cycle is repeated and all emissions measurements are taken as described for FTP testing with the exception that a single bag is used to collect the dilute exhaust sample (single phase). Fuel economy, in miles per gallon, is again determined via the carbon balance method. The Phase 2 testing protocol added two additional repeats for the HwFET cycle that were run and measured sequentially.

US06 Schedule Testing This test type is the aggressive-driving portion of the supplemental FTP (SFTP), consisting of higher speeds and acceleration rates.

SC03 (AC2 Alternate) Schedule Testing This test type has been introduced to represent the engine load and emissions associated with the use of air conditioning units in vehicles. Since the TRC, Inc. emissions lab lacks the solar-loading equipment necessary to run a full SC03 test, the AC2 alternative was used. This alternative was only valid for 2000-2001 model year vehicles unless approved by the EPA, therefore the result for each individual cycle is reported in this report but not composite 5-cycle numbers for the vehicle.[23] The AC2 alternative mimics the SC03 except that the thermal load is simulated by placing the vehicles air conditioning temperature control to full hot, air conditioning on, and the drivers side window left down. In addition, the test cell is kept at 76 F and 50 grains of water per pound of dry air versus the SC03 requirement of 95 F and 100 grains of water per pound of dry air. All other procedures follow the SC03.

Cold CO Schedule Testing This test follows the same driving cycle as the FTP, but the test is performed at 20 F and the vehicle is filled with Cold CO specific fuel. The vehicle is operated through one UDDS preparation cycle at 20 F. Then, the vehicle is parked in a soak chamber maintained at 20 F for a minimum of 12 and a maximum of 36 hours prior to beginning each test. Following the 20 F. soak, the vehicle is pushed into the dynamometer chamber (which is at 20 F) and then operated through the normal FTP test.

The program was completed in blocks of tests, with the M14 control tires and G12 OE tire run multiple times to track possible vehicle, tire and test equipment drift. The completed test cycles are summarized in Table 7.

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Table 7. Fuel Economy Test Schedules Pressure City (FTP) Highway (HwFET)* High Speed (US06) AC (SC03) Cold

Temp (Cold CO) 210 kPa 19 57 19 19 19 158 kPa 6 16 6 6 6

*Two extra cycles completed after first run to gauge statistical variability.

2.8 Skid-Trailer Tire Traction Testing

FMVSS No. 575.104, Uniform tire quality grading standards requires manufacturers to provide a (wet slide) traction grade for all tires subject to standard and manufactured after April 1, 1980. A formal description follows[24]:

To assist consumers purchasing new vehicles or replacement tires, NHTSA has rated more than 2,400 lines of tires, including most used on passenger cars, minivans, SUVs and light pickup trucks. Traction grades are an indication of a tire's ability to stop on wet pavement. A higher graded tire should allow a car to stop on wet roads in a shorter distance than a tire with a lower grade. Traction is graded from highest to lowest as "AA", "A", "B", and "C". Of current tires: 3 percent are rated AA, 75 percent are rated A, 22 percent are rated B, only 1 line of tires rated C.

The UTQGS skid-trailer traction testing was performed at the NHTSA test facility on Goodfellow Air Force Base in San Angelo, Texas. The traction grading tests are now performed on a purpose-built oval at the base rather than the original test surface diagram shown in 575.104. The test pavements are asphalt and concrete skid pads constructed in accordance with industry specifications for skid surfaces. ASTM E 5014 reference (control) tires are used to monitor the traction coefficient of the two surfaces (which varies based on environmental conditions, surface wear, etc.). During a normal wet traction test, a vehicle tows a skid-trailer (Figure 10) at 40 mph across the test surfaces. Water is dispersed ahead of the tire from a water nozzle just before the brake is applied. Instrumentation measures the horizontal force as the brake is applied to one wheel of the trailer until lock-up, and then held for a few seconds and released. The tests are repeated for a total of 10 measurements on each surface. The candidate (test) tires are conditioned by running for 200 miles on a pavement surface. The candidate tires are then fitted to the trailer, loaded to a specified load and pressure, then subjected to the same testing completed on the control tires. The average sliding coefficient of friction for the candidate tire on each surface is corrected using the coefficients of the control tire to yield an adjusted traction coefficient for the candidate tire on each test surface.

4 ASTM E 501-94 Standard Specification for Standard Rib tire for Pavement Skid Resistance Tests. Available from American Society for Testing and Materials, http://astm.org.

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http:http://astm.org

Figure 10. NHTSA San Angelo Skid-Trailer

Phase 2 traction tests were conducted with tires of 16 models previous tested in Phase 1. Two tires had the highest traction grade AA, 14 tires were graded A (Table 8). Since these tires experienced some break-in during the 50- to 70-mile rolling resistance tests, these tires were only conditioned for 70 miles on a pavement surface rather than the normal 200 miles.5 Since the tires were not new, and had a reduced break-in, the results generated are for research purposes and are unofficial. The test matrix was also repeated on dry asphalt and concrete test surfaces. The number of measurements on the dry surfaces was reduced to preserve the limited test surface area from rubber buildup.

Since modern antilock brakes (ABS) and electronic stability control (ESC) operate in the lower slip and higher friction region, the peak coefficient recorded during the traction testing was also used for comparisons in Phase 2 in addition to the slide values used for UTQGS wet traction.

5 Two additional tires of a Phase 1 tire model were broken -in for the full 200 miles and compared to a set of two that had the 50- to 70-mile roadwheel break-in. There was no significant difference in their traction numbers.

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Table 8. Phase 2 Wet and Dry Skid-Trailer Test Tires Ti

re M

odel

Cod

e

MFG

Size

Load

Inde

x

Spee

d R

atin

g

Mod

el

UTQ

GS

Trea

d-w

ear

UTQ

GS

Trac

.

UTQ

GS

Tem

p.

Perf

orm

ance

Le

vel

ISO

285

80 R

ollin

g R

esis

tanc

e, F

r (lb

f)

Wei

ght (

lbs.

)

B14 Bridgestone P225/60R16 97 V Turanza LS-V 400 AA A Grand Touring All Season 13.90 28.6

B12 Bridgestone P225/60R16 98 W Potenza RE750 340 AA A Ultra High Performance Summer

15.22 27.4

D10 Cooper 225/60R16 98 H Lifeliner Touring SLE 420 A A Standard Touring All Season 13.56 25.2

P5 Pep Boys (Cooper)

P225/60R16 97 H Touring HR 420 A A Passenger All Season 14.02 25.7

R4 Pirelli 225/60R16 98 H P6 Four Seasons 400 A A Passenger All Season 14.98 24.3

B11 Bridgestone P225/60R16 97 H Potenza RE92 OWL 340 A A High Performance All Season 10.13 25.1

M13 Michelin 225/60R16 98 H Pilot MXM4 300 A A Grand Touring All Season 12.07 24.7

B13 Bridgestone P225/60R16 97 T Turanza LS-T 700 A B Standard Touring All Season 15.01 29.4

M14 Uniroyal P225/60R16 97 S ASTM 16" SRTT 540 A B ASTM F 2493-06 Reference 11.96 25.5

G8 Goodyear 225/60R16 98 S Integrity 460 A B Passenger All Season 9.83 22.9

G11 Goodyear P225/60R17 98 S Integrity 460 A B Passenger All Season 10.02 24.5

G9 Goodyear P205/75R14 95 S Integrity 460 A B Passenger All Season 11.27 19.2

G10 Goodyear P205/75R15 97 S Integrity 460 A B Passenger All Season 12.09 20.4

U3 Dunlop (Sumitomo)

P225/60R17 98 T SP Sport 4000 DSST 360 A B Run Flat 13.91 36.4