Low Rolling Resistance Tires
Rolling resistance is fundamentally the parasitic energy a tire consumes while rolling under load. 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 (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 0.1-0.2. A Green Seal report, Low Rolling Resistance Tires, indicates that a 1.5%-4.5% savings is possible for high rolling resistance tires. The same 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 a very low rolling resistance tire.
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.
Proper tire inflation also plays a large role in keeping vehicles fuel efficient. For example, a vehicle with a recommended pressure of 35 psi whose tires are at 28 psi will have increased its rolling resistance by 12.5%.
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. For more information visit the California Energy Commission's Fuel Efficient Tire Proceeding Documents Web page.
|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 cyle with aggressive highway driving.