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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286
KEY FINDINGS AND ESTIMATES
Rolling resistance has a meaningful effect on vehicle fuel consumption.
For conventional passenger vehicles, most of the energy contained in a gallon of motor fuel is lost as heat during engine combustion and from friction in the driveline, axles, and wheel assemblies. Some of the energy produced by the engine is consumed during idling and by vehicle accessories. Only about 12 to 20 percent of the energy originating in the fuel tank is ultimately transmitted to the wheels as mechanical energy to propel the vehicle. Rolling resistance consumes about one-third of this transmitted energy.
In one sense, rolling resistance consumes only a small fraction of the total energy extracted from a gallon of fuel. In another sense, a reduction in rolling resistance will reduce demand for mechanical energy at the axles. This will have a multiplier effect because it will translate into fewer gallons of fuel being pumped to the engine in the first place.
The overall effect of a reduction in rolling resistance on vehicle fuel economy will depend on a number of factors, including the underlying efficiency of the engine and driveline as well as the relative amounts of energy consumed by other factors, such as aerodynamic drag and vehicle accessories. For most passenger vehicles, a 10 percent reduction in rolling resistance will have the practical effect of improving vehicle fuel economy by about 1 to 2 percent.
Tires are the main source of rolling resistance.
The rolling resistance encountered by a vehicle can be extreme when it is driven on a soft or rough surface, such as a gravel or dirt road. On hard paved surfaces, which are more common for the operation of passenger vehicles, the main source of rolling resistance is the repeated flexing of the vehicle’s tires as they roll. Through an effect known as hysteresis, this repeated flexing causes mechanical energy to be converted to heat. More mechanical energy must be supplied by the engine to replace the energy lost as heat from hysteresis. The design, construction, and materials of tires, as well as their maintenance, their condition, and operating conditions, affect the rate of energy loss. For most normal driving, a tire’s rolling resistance characteristics will not change in response to an increase or decrease in vehicle travel speed.