Tires and Passenger Vehicle Fuel Economy Informing Consumers, Improving Performance -- Special Report 286 (2006) / Chapter Skim
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3 The Tire’s Influence on Passenger Vehicle Fuel Consumption
3 The Tire’s Influence on Passenger Vehicle Fuel Consumption
Pages 36-76
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From page 36... ...
While a full accounting of such life-cycle effects is relevant for policy making, it would have exceeded the scope and capabilities of this study. The chapter begins with a review of the history of interest in vehicle fuel economy and the effect of tires on fuel consumption.
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program.1 For the first time, Congress established fuel economy standards for passenger cars and light trucks. The program, administered by the National Highway Traffic Safety Administration (NHTSA)
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It is notable, however, that NHTSA and EPA are revising the long-standing means of measuring and calculating vehicle fuel economy, which could eventually affect the implementation of CAFE. 2EPA is responsible for providing fuel economy data that are posted on the window stickers of new vehicles.
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From page 39... ...
While the specific percentages will vary by vehicle type and trip, the flows shown in Figure 3-1 are generally representative of passenger vehicles today. For both urban and highway driving, the mechanical energy that does make its way through the driveline to turn the wheels is consumed by three sinks: aerodynamic drag, rolling resistance, and braking.
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highway driving.
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A related issue is that improvements in vehicle fuel economy have the secondary effect of increasing vehicle travel. As vehicle fuel economy improves, the per-mile cost of driving is effectively lowered, which may spur some additional driving and fuel consumption.
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From page 42... ...
Maintaining proper tire inflation and alignment is important for motor vehicle safety as well as for fuel economy; this is true for all pneumatic tires regardless of their design. This section therefore focuses on designing tires with lower rolling resistance when properly inflated.3 It has long been known that a rolling tire must be supplied energy continuously in order to avoid losing speed.
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On paved surfaces that deflect very little under the loads of passenger cars, tire deformation is the main source of hysteresis.
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In comparison with the bias-ply tire, the steel-belted radial tire reduced the deformation of the tread in the contact patch. Hence, in addition to affecting tire handling, endurance, and ride comfort, the changeover from bias-ply to radial-ply tires during the 1970s and 1980s reduced tire rolling resistance by an estimated 25 percent without requiring major changes in the polymers used (Schuring 1980, 601)
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Tire Operating Variables and Hysteresis A number of tire operating conditions affect rolling resistance. The most important are load, inflation pressure, and temperature.
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Schuring (1980) observes that for conventional passenger tires, an increase in inflation pressure from 24 to 29 pounds per square inch (psi)
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From page 47... ...
MEASURING AND EXPRESSING ROLLING RESISTANCE The fact that rolling resistance relates linearly to wheel load allows it to be expressed as a near-constant coefficient relative to wheel load. The rolling resistance coefficient (RRC)
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J2452 was developed later to provide additional quantification of a tire's rolling resistance for more precise inputs to the driving cycles used for federal vehicle emissions and fuel economy regulatory compliance. The speed-adjusted measurements generated from J2452 can be entered into simulated driving cycles, such as those used for testing new vehicles for CAFE compliance.
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From page 49... ...
The most common way to describe this relationship is by relating the percentage change in RRC to the percentage change in fuel economy; for example, "a 10 percent change in RRC yields a 2 percent change in vehicle fuel economy." This approach is generally acceptable for the relatively narrow range of RRCs observed for most passenger tires. However, it can introduce imprecision, since a given percentage change in fuel economy is linearly related to an absolute change in rolling resistance.
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For instance, Thompson and Reineman (1981) , in assisting EPA with the development of fuel economy models, assumed that a change of 0.001 in RRC would change vehicle fuel consumption by 1 percent during urban driving and 2 percent during highway driving.
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The results of literature reviews and the output of these simulations are sufficiently consistent to estimate a response range for RRC that is meaningful for most driving patterns and common types of passenger vehicles. They are consistent with the long-standing rule of thumb that a 10 percent reduction in RRC will yield a 1 to 2 percent increase in vehicle fuel economy.
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From page 52... ...
Of most significance, rolling resistance measurements for more than 150 new passenger tires were made publicly available by three major tire companies during the course of this study. While the data set has limitations, it contains data on many new tires currently on the market and supplemental data on each tire's speed rating, size, traction and tread wear ratings, tread depth, and retail price.
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Michelin and Other Tire Company Data Submitted to NHTSA Rulemaking (19941995) In 1994, when NHTSA proposed adding a fuel economy label for passenger tires as part of the Uniform Tire Quality Grading (UTQG)
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In addition to the 36 radial tire lines, EPA tested 18 bias-ply tire lines. The bias-ply tires (including some bias-belt tires)
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From page 55... ...
In other comments to NHTSA in the same rulemaking, Goodyear provided its own estimates of the range of RRCs commonly found among OE and replacement tires. It estimated ranges of 0.0067 to 0.0152 for new OE tires and 0.0073 to 0.0131 for new replacement tires, although it did not name the tires included.11 EPA Coastdown and Fuel Economy Tests (2001)
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From page 56... ...
56 Tires and Passenger Vehicle Fuel Economy TABLE 3-3 OE and Replacement Passenger Tire RRCs Measured for Tires When New, Reported by Michelin in 1994 and 1995 Brand Tire Line RRC OE Tire Measurements Reported in 1994 Goodyear Invicta GLR 0.0073 Dunlop SP23V 0.0077 Michelin XW4 0.008 Michelin LXI 0.0088 Firestone FR680 0.0094 Michelin XGT4 0.0098 Michelin MX4 0.01 Firestone Supreme 0.0105 Firestone FR480 0.0105 Mean 0.0091 Median 0.0094 OE Tire Measurements Reported in 1995 Michelin XW4 P195/70/R14 S 0.0077 Michelin MXV4 P205/60/R16 H 0.0078 Uniroyal Tiger Paw AWP P155/60/R13 S 0.008 Michelin XW4 P215/70/R15 S 0.0082 Michelin MX4 Green X P195/65/R15 S 0.0084 Michelin XW4 P195/70/R14 S 0.0084 Uniroyal Tiger Paw AWP P215/65/R16 0.0087 Michelin Energy MX4 P235/60/R15 H 0.0088 BF Goodrich Touring T/A P205/70/R15 S 0.0088 Uniroyal Tiger Paw AWP P205/75/R15 S 0.0089 Michelin MXV4 P205/60/R15 V 0.009 BF Goodrich Touring T/A P195/65/R15 S 0.009 Michelin MXV4 P155/60/R13 S 0.009 BF Goodrich Touring T/A P205/70/R15 H 0.0091 Michelin XW4 P215/65/R15 S 0.0093 Michelin MXV4 P205/65/R15 0.0095 BF Goodrich Touring T/A P175/70/R14 S 0.0097 Michelin Energy MX4 P195/65/R15 H 0.0098 Michelin MXV4 P205/60/R15 H 0.0099 Michelin XW4 P225/60/R16 S 0.01 Michelin MXV4 P215/65/R16 T 0.0103 Michelin MXV4 P165/65/R15 0.0105 Michelin Energy MX4 P185/65/R14 H 0.0107 Uniroyal Tiger Paw AWP P145/60/R15 T 0.0114 Mean 0.0092 Median 0.009
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The Tire's Influence on Passenger Vehicle Fuel Consumption 57 Brand Tire Line RRC Replacement Tire Measurements Reported in 1994 Goodyear Invicta GL 0.0087 Goodrich Momenta S/E 0.0095 Michelin MXL 0.0097 Cooper Cornell 800 0.0098 Kelly Kelly Explorer 400 0.01 UG Cientra 0.01 Goodrich Touring T/A 0.01 Uniroyal Tiger Paw A/S 0.01 Kelly Charger 0.0102 Kleber CP75 0.0103 UG Defender SRX +4 0.0104 Goodrich Radial T/A 0.0105 Cooper Trendsetter II A/W 0.0105 Uniroyal Rally GTS 0.0105 Michelin XGTH4 0.0107 Goodrich Lifesaver A/W 0.0107 Kelly Voyager 1000 Touring 0.0109 Goodrich The Advantage 0.011 Cooper Lifeliner Classic 0.011 Kelly Navigator 800S 0.0112 Uniroyal Tiger Paw XTM 0.0112 Cooper Monogram A/W 0.0113 UG UG Liberator II+ 0.0113 Goodrich Tour T/A 0.0114 Armstrong Sears Guardsman 0.0116 Cooper Cobra GTS 0.0117 Yokohama Y376A 0.0118 Uniroyal Tiger Paw 0.012 Michelin XGTH4 0.0121 Firestone FTX 0.0121 Goodyear Aquatred 0.0122 Goodyear Eagle GA 0.0124 Firestone FTX 0.0127 Sumitomo HTR4 0.0127 Michelin MX4 0.0134 Goodyear Eagle GA 0.0137 Dunlop D60A2 0.0143 Mean 0.0112 Median 0.0103 (continued on next page)
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In 2001, it conducted load and fuel economy tests on several tires installed on the same vehicle (Automotive Testing Laboratories 2002)
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Meanwhile, measurements of vehicle fuel economy for the Camry were higher by 1.3 to 10.4 percent for four of the five replacement tires. Ecos Consulting Data (2002)
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60 Tires and Passenger Vehicle Fuel Economy TABLE 3-4 Rolling Resistance Coefficients for 34 Passenger Tires, Measured When New by Ecos Consulting in 2002 Tire Manufacturer Tire Line Size RRC Bridgestone/Firestone B381 P185/70/R14 0.0062 Continental Ameri-G4S WS P235/75/R15 0.0078 Goodyear Invicta GL P235/75/R15 0.0081 Continental ContiTouring Contact CH95 P205/55/R16 0.0083 Uniroyal Tiger Paw AWP P185/70/R14 0.0088 Michelin Energy MXV4 Plus P205/55/R16 0.009 Goodyear Eagle RS A P205/55/R16 0.0092 Bridgestone/Firestone Long Trail T/A SL P245/75/R16 0.0092 Michelin Pilot Sport Cup P205/55/R16 0.0092 Sumitomo HTR 200 P185/70/R14 0.0092 Pirelli P6000 P205/55/R16 0.0095 General Grabber AP SL P235/75/R15 0.0097 Goodyear Integrity P185/70/R14 0.0097 Bridgestone/Firestone FR680 WS P235/75/R15 0.0102 Dunlop SP40 A/S P185/70/R14 0.0103 Michelin LTX M/S P245/75/R16 0.0103 Bridgestone/Firestone Dueler A/T D693 P245/75/R16 0.0103 Bridgestone/Firestone Wilderness AT P235/75/R15 0.0105 Kumho Venture AT P245/75/R16 0.0105 Bridgestone/Firestone Potenza RE92 P185/70/R14 0.0107 Michelin Harmony P185/70/R14 0.0107 Goodyear Regatta 2 P185/70/R14 0.0108 Michelin Symmetry P185/70/R14 0.0108 Bridgestone/Firestone Turanza LS-H P205/55/R16 0.0109 Bridgestone/Firestone Turanza LS-T P185/70/R14 0.0109 Bridgestone/Firestone Affinity Touring P235/75/R15 0.011 Michelin Pilot Sport P205/55/R16 0.0111 Goodyear Eagle F1 GS-D3 P205/55/R16 0.0112 Dunlop SP Sport A2 SL P205/55/R16 0.0113 Goodyear Aquatred 3 P185/70/R14 0.0113 Goodyear Conquest AT P245/75/R16 0.0114 Bridgestone/Firestone Firehawk SZ50EP P205/55/R16 0.012 Goodyear Eagle GT II P205/55/R16 0.0121 Michelin Pilot Sport A/S P205/55/R16 0.0133 Mean 0.0102 Median 0.0104 SOURCE: Ecos Consulting, personal communication, August 2005.
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From page 61... ...
Of the 40 tires tested, the rolling resistance of 21 was characterized as excellent or very good, 15 as good or fair, and 4 as poor. Consumer Reports stated that the difference in vehicle fuel economy (miles per gallon)
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From page 62... ...
All of the OEMs reported measuring and specifying rolling resistance under the SAE J2452 test procedure because the results can be fitted into models for the federal driving cycles used in emissions and fuel economy testing. Achieving federal emissions and fuel economy targets is a major reason why OEMs are concerned with rolling resistance.
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From page 63... ...
Another OEM provided the following new-tire rolling resistance ranges for similar tire categories, which were derived by using the SAE J2452 test procedure and reported by using the Standard Mean Equivalent Rolling Force conditions described in the Appendix: · All-season, 0.005 to 0.0062; · Touring, 0.0058 to 0.0075; · Performance, 0.0065 to 0.0083; and · High performance, 0.009. The company did not provide typical rolling resistance values for light truck passenger tires.
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From page 64... ...
One OEM reported that tires installed on hybrid vehicles are generally not specified any differently from those installed on nonhybrid cars designed to achieve high fuel economy. Another noted that the attention given to tire rolling resistance can be expected to increase with the advent of hybrid drivetrains and technologies such as cylinder cutout, since the fuel economy effects are greater.
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From page 65... ...
b (lb) <20 21 13.6 2022 31 20.1 2325 32 20.8 2630 28 18.2 3135 23 14.9 36+ 23 14.9 NOTE: Replacement tires in sample = 154; OE tires in sample = 8; total tires = 162.
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From page 66... ...
In this respect, it offers many opportunities for analyzing rolling resistance levels and relationships with respect to other attributes such as wear resistance, traction, size, selling price, and speed ratings. To expand these analytic opportunities, the committee supplemented the information provided by the tire companies with publicly available data on each tire's tread depth, weight, and retail prices obtained from manufacturer and tire retailer websites.
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From page 67... ...
13The committee cannot know how many of the replacement tires in the data set were originally de veloped for the OE market or are still being used for some OE applications.
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From page 68... ...
This pattern suggests a relationship between RRC and speed rating. However, performance tires are more likely to have lower aspect ratios, wider section widths, and larger rim diameters than tires with lower speed ratings.
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From page 69... ...
The Michelinreported data for replacement tires on the market in the mid-1990s show further progress in reducing rolling resistance, especially in the number of tires achieving RRCs below 0.01. The most recent data, from Ecos Consulting in 2002 and RMA in 2005, reveal additional reductions in the average and median rolling resistance.
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These values are estimates by OEMs on the basis of the SAE J2452 test procedure. See the Appendix for an explanation and comparison of the two SAE rolling resistance test procedures.
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From page 71... ...
Today's replacement tires -- as represented in the 2002 and 2005 data sets -- include many high-performance tires. These tires, with speed ratings of W, Y, and Z, account for a disproportionate share of tires with high RRCs, as shown in Figure 3-5.
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From page 72... ...
Many of the S and T tires that have higher RRCs in the 2002 and 2005 data possess rim diameters of 13 and 14 inches. EPA only tested tires with 15-inch rim diameters.
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From page 73... ...
The effect on total fuel consumption will depend on a number of factors, including the efficiency of the engine and driveline as well as the amount of energy used to power accessories. For most passenger vehicles, a 10 percent reduction in average rolling resistance over a period of time will lead to a 1 to 2 percent reduction in fuel consumption during that time.
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Federal fuel economy standards have prompted automobile manufacturers to demand OE tires with lower rolling resistance. Information on precisely how these lower-rolling-resistance characteristics have been achieved is proprietary.
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From page 75... ...
2002. Coastdown and Fuel Economy for Specific Vehi cles and Tires.
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From page 76... ...
1994. Effects of Tire Rolling Loss on Vehicle Fuel Consumption.
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