Treading Lightly for Better Fuel Economy

Jan. 10, 2008
The treads on your car’s tires may be getting thinner, and not because you’re racking up the miles.

Thinner treads translate to lower rolling resistance and better fuel economy. Automakers are catching on to this as they strive to meet EPA regulations and demands for fuel efficiency.

A 2006 National Academy of Sciences study concluded that a 10% decrease in rolling resistance can boost fuel economy 1 to 2%. For the average driver, that means saving 6 to 12 gallons of gas yearly.

Lower rolling resistance improves fuel economy by cutting the energy lost through tire deformation and recovery. The lost energy, dissipated as heat between the tire and the road, consumes up to 4 to 7% of the energy in the vehicle’s fuel. About 7 hp gets consumed for every 40 lb of rolling resistance at 60 mph.

A frequently used measure is the coefficient of rolling resistance (CRR), the rolling resistance divided by wheel load (vehicle weight divided by four for most passenger vehicles). Most CRRs fall between 0.007 and 0.014.

Rolling resistance gradually drops over the life of any tire as treads wear. But if you buy new tires with thinner treads, you might have to replace them sooner. More-frequent replacement leads to more scrap tires and more resources used making new tires. The NAS report postulates that any savings seen from better fuel economy would be erased by as little as a 5% shorter tire life.

Tire manufacturers acknowledge the need to slow tread wear. “We want to be able to maximize fuel efficiency, tread wear, and traction,” says Dan Zielinski of the Rubber Manufacturers Association, which represents the tire industry in Washington, D.C. Because each attribute affects the others, “no one [tire] will be the best in all three categories.”

Manufacturers also stress the role of tire inflation. Even on tires with thinner tread, underinflation can wipe out any gains in fuel economy. Each psi under the recommended inflation bumps up the rolling resistance by 1.4%. Underinflating by a visually undetectable 5 to 7 psi leads to 10% more fuel consumption.

Making tires with stiffer rubber compounds is another way to trim rolling resistance. Harder rubbers have a smaller hysteresis loop, so they deform less as the tire rolls and need less energy to recover from deformation.

Stiffer rubber compounds also wear more slowly and resist heat better than more compliant rubbers. However, stiffer tires can have less traction and take longer to stop, especially in wet conditions.

Tiremakers have already replaced portions of carbon-black filler in their tires with silica particles treated with organosilanes. These silica-silanes reduce wear and rolling resistance while providing slightly better traction in snow.

The silica particles cost about 66% more than carbon black, however. Current tires seldom contain more than one part silica- silane to every two parts carbon black.

Other changes to tire materials and construction may be on the horizon. Research and development efforts will most likely depend on whether customers demand long-wearing, low-rollingresistance tires.

The U.S. Congress recently considered an energy bill requiring the National Highway Transportation Safety Administration to create a system to educate customers about the relative fuel efficiency of replacement tires at the point of sale. The NHTSA rating would be based on tire rolling resistance under proper inflation. Tiremakers would do the testing, but the test method has yet to be determined. According to the RMA, manufacturers would need to invest in test equipment that is not now widely available.

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