Low Rolling Resistance Tires
Rolling resistance is the energy lost from drag and friction of a tire rolling over a surface. The phenomenon is quite complex, and nearly all operating conditions can affect the final outcome. It is estimated that 5%-15% of light-duty fuel consumption is used to overcome rolling resistance for passenger cars. For heavy trucks, this quantity can be as high as 15%-30%.
A modeling study conducted at the National Renewable Energy Laboratory, Analysis of the Fuel Economy Benefit of Drivetrain Hybridization, shows the sensitivity coefficient (the percent increase in fuel economy per percent decrease in rolling resistance) to be as high as 0.2 for a conventional vehicle. The current study quantifies this number by duty cycle and shows a range of 0.1 to 0.2. In addition, a Green Seal report quotes testing done for rolling resistance on tires with rolling resistance coefficients varying from ~0.01 all the way down to ~0.006, implying up to 3% improvement in fuel economy for low rolling resistance tires.
New vs. Replacement Tires
New cars are generally equipped with low rolling resistance tires that offer better fuel economy. This helps the auto manufacturer to meet their Corporate Average Fuel Economy (CAFE) standards. However, no requirements are currently placed on replacement tires. Therefore, if you want to purchase fuel-efficient replacement tires, you must research to figure out which tires have low rolling resistance.
Signed into law on December 4, 2015, the Fixing America’s Surface Transportation (FAST) Act requires the U.S. Department of Transportation, U.S. Environmental Protection Agency, and U.S. Department of Energy to develop regulations for passenger vehicle tire fuel efficiency standards by December 2017. Some exemptions apply. For more information, refer to H.R. 22, Section 24331.
Proper tire inflation plays an important role in fuel efficiency. Just by keeping your tires properly inflated, you can improve your gas mileage by as much as 3.3%. Under-inflated tires can lower gas mileage by 0.3% for every 1 pressure per square inch (psi) drop in pressure of all four tires. To find the proper tire pressure for your vehicle, check the sticker on the driver’s side door or the owner’s manual.
According to a California Energy Commission report, the use of low rolling resistance tires on light-duty fleets is cost effective over the life of the vehicle. That is, the fuel savings pays for the additional cost of the low rolling resistance tires. This report finds that the opportunity for cost-effective energy savings in California from low rolling resistance tires is substantial—about 300 million gallons of gasoline per year resulting from a 3% average improvement in the fuel efficiency of light-duty vehicles currently operating on replacement tires.
|Additional fuel burned per 1000 miles for poor inflation of baseline tire (in gallons)||0.6||0.5||0.6||0.5||0.5||0.5||0.5|
|Fuel savings potential per 1000 miles for a 10% reduction in rolling resistance (in gallons)||0.5||0.4||0.5||0.4||0.4||0.4||0.4|
|Percent increase in fuel consumption for poor inflation of baseline tire||2.30||1.49||1.51||1.14||1.08||1.20||0.94|
|Percent decrease in fuel consumption per percent decrease in rolling resistance||0.19||0.12||0.13||0.09||0.09||0.10||0.08|
|Average speed for the overall duty cycle [mph]||48||48||21||21||25||20||14|
This can be seen graphically as follows:
In the equation, it is assumed that the baseline rolling resistance is 0.009. RRCreplacement is the rolling resistance of the replacement tire which can be obtained from sources such as Green Seal's Choose Green Report, March 2003. Negative values indicate an increase in fuel consumption.
Relevant Information and Reports
California Energy Commission's Fuel Efficient Tire Proceeding Documents Web page
Fuel Economy.Gov - Advanced Technologies and Energy Efficiency
Analysis of the Fuel Economy Benefit of Drivetrain Hybridization, National Renewable Energy Laboratory
1015 Driving Cycle
A Japanese emissions test cycle characterized by slow speed urban driving.
Highway Federal Emissions Test (HWFET)
Highway federal emissions test (the "highway" cycle for the EPA city/highway test procedure). A very gentle high-speed cycle.
LA92 Drive Cycle
1992 test data from Los Angeles that consists of city/highway mix and can be characterized by aggressive urban driving.
New European Drive Cycle (NEDC)
The New European Drive Cycle (NEDC) is a European Union (EU) test cycle characterized by a city/highway driving mix.
SC03 Drive Cycle
This is supplemental drive cycle number 3 for the federal test procedure. It consists of a city/highway mix of driving.
Urban Dynamometer Driving Schedule (UDDS)
The urban dynamometer driving schedule is characterized by gentle urban driving.
US06 is defined as a duty cycle with aggressive highway driving.