|Energy flow for a modern mid-size sedan. Rolling resistance consumes about one-third of the mechanical energy output from the engine to the wheels.|
Two divisions of the National Academies—the Transportation Research Board and the Board on Energy and Environmental Systems—have issued a special report examining the contribution of tires to vehicle fuel consumption and the prospects for improving tire energy performance without adversely affecting tire life, traction capability, and retail prices.
Tires and Passenger Vehicle Fuel Economy concludes that (1) reducing the average rolling resistance of replacement tires by a magnitude of 10% is technically and economically feasible and (2) such a reduction promises a 1% to 2% increase in fuel economy—representing between 1 to 2 billion gallons of fuel saved.
Tires affect vehicle fuel economy mainly through rolling resistance. As a tire rolls under the vehicle’s weight, its shape changes repeatedly as it experiences recurring cycles of deformation and recovery.
In the process, mechanical energy otherwise available to turn the wheels is converted into heat and dissipated from the tire. More fuel must be expended to replace this lost energy.
Combinations of differences in tire dimensions, design, materials, and construction features will cause tires to differ in rolling resistance as well as in many other attributes such as traction, handling, noise, wear resistance, and appearance.
Once they are placed in service, tires must be properly maintained to perform as intended with respect to all attributes. The maintenance of proper inflation pressure is especially important.
Although reducing the average rolling resistance of the tires may be feasible technically and economically, the report also concludes that consumer understanding and behavior is a major obstacle.
The committee does not underestimate the challenge of changing consumer preferences and behavior. This could be a difficult undertaking, and it must begin with information concerning the tire’s influence on fuel economy being made widely and readily available to tire buyers and sellers.
A significant and sustained reduction in rolling resistance is difficult to imagine under any circumstances without informed and interested consumers.
The committee observes that consumers now have little, if any, practical way of assessing how tire choices can affect vehicle economy.
The main source of rolling resistance in on-road driving is the repeated flexing (deforming and recovery) of the tires as they roll. Through an effect known as hysteresis, this repeated flexing causes mechanical energy to be converted to heat. This is turn, requires the application of more mechanical energy from 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—although not an increase or decrease in travel speed—affect the rate of energy loss.
To improve traction and prolong wear, the tread component of the tire must have a substantial portion of the deformable, hysteretic material in the tire. The type and amount of material in the tread are therefore important determinants of rolling resistance. Other tire features and design parameters affect rolling resistance as well, including tire mass, geometry, and construction type.
According to the report, the large majority of new passenger tires, properly inflated, have rolling resistance coefficients (RRC) ranging from 0.007 to 0.014, with most having values closer to the average of about 0.01.
|The distribution of RRCs across the tires examined for the report.|
The spread in rolling resistance values has increased of late, however, due to proliferating sizes, types and speed capabilities. Tires with high-speed ratings—which have become more popular in the replacement market—tend to have higher-than-average rolling resistance.
The committee concludes that its 10% reduction in average rolling resistance could occur through a combination of changes in the distribution of tires purchased and greater use being made of various technologies (materials and design) to reduce rolling resistance. It could also be achieved in part through more vigilant tire maintenance.