By Jennifer Barker, EORenew www.solwest.org

If you own a hybrid or other high gas-mileage vehicle, have you replaced the original tires and then been disappointed by a reduction in fuel efficiency? Last spring, I replaced the original equipment (OE) tires on my 40+ mpg Honda Civic HX. Afterwards, I found myself commiserating with my hybrid-owning friends who had also replaced their original tires, and been shocked to find their gas mileage substantially decreased. My own record-keeping showed a 2 mpg, or 5%, loss in gas mileage. My experience and anecdotal reports from friends indicate that a vehicle which is designed for fuel efficiency may lose 5-10% of its gas mileage, or 2-6 mpg, for a car that achieved 40-60 mpg with its OE tires.

What is Rolling Resistance?

Most of the energy contained in a tank of motor fuel is dissipated as unrecoverable heat from engine combustion and friction in the driveline. Accessories, aerodynamic drag, and braking consume some, too. Only about 12 to 20 percent of the energy produced by the engine actually turns the wheels, and about one-third of that is consumed by rolling resistance. As you drive down the road, your tires squirm and deform every time you go around a corner, accelerate, or apply the brakes. The repeated tire deformation between the tire and the road, within the tire and between the tire and the rim, causes mechanical energy to be converted to heat; hence additional mechanical energy must be supplied to drive the axle.

The design characteristics of a tire that affect this energy loss are its construction; geometric dimensions; and materials types, formulations, and volume. The tread, in particular, plays a major role because it contains large amounts of viscoelastic rubber material. A tire’s rolling resistance also increases in proportion to the wheel load.

How Does Rolling Resistance Affect Gas Mileage and Performance?

In the end, rolling resistance consumes only a small portion of the total energy expended by the vehicle. A reduction in rolling resistance will reduce demand for mechanical energy at the axles. This has a multiplier effect because it translates into fewer gallons of fuel being pumped to the engine in the first place. The effect on total fuel consumption will depend on a number of factors, including the efficiency of the engine and driveline as well as the amount of energy used to power accessories. For most passenger vehicles, industry figures estimate a 10 percent reduction in average rolling resistance over a period of time will lead to a 1 to 2 percent reduction in fuel consumption during that time (though my records and anecdotal reports indicate that ultra-efficient vehicles lose more).

It’s important to note the new trend toward bigger tires has affected fuel economy because, all else being equal, bigger rims and wider tires are heavier, less aerodynamic and create more rolling resistance. Run-flat tires, which are becoming more popular, appear to have at least 20 percent higher rolling resistance (in general) than the conventional OE tires supplied on the same vehicle, in part because run-flat tires have additional structural material and mass. Attention given to tire rolling resistance can be expected to increase with the advent of hybrid drive trains and technologies such as cylinder cutout, since the fuel economy effects are greater.

As a simple rule, the harder a tire is the less rolling resistance it will have, which is why making sure your tires are properly inflated is so important. The more a tire is loaded at a given pressure, the more it deforms and suffers hysteretic losses. A tire deforms more when it is under-inflated. For tires inflated to pressures of 24 to 36 psi, each 1-psi drop in inflation pressure increases the tire’s rolling resistance by about 1.4 percent. The effect is greater for inflation pressures below 24 psi. Consequently, maintenance of tire pressure is important for a tire’s energy performance as well as for tire wear and operating performance. Some tire experts recommend inflating a few pounds higher than your car manufacturer’s specifications, but adding too much extra pressure to tires can cause them to wear prematurely.

The relationship between rolling resistance and wear resistance has been found to be determined by a combination of factors, including the amount of materials in the tread, the chemistry of the rubber and the mechanical design and dimensions of the tire. Some industry insiders suggest that forcing reductions in rolling resistance could have a negative impact on tire grip and tread life. Performance is often, but not always, related to rolling resistance. Likewise, tread-life is usually, but not always, shorter in low-rolling-resistance tires. Shorter tire wear life results in more scrap tires and in consumers spending more on tire replacement, both of which are undesirable. Consequently, tire companies and their material suppliers have invested in research and development to find ways to reduce rolling resistance with minimal adverse effects on tread wear.

Yokohama, Cooper and Continental are introducing new LRR tires in 2009. Jim Micali, president of Michelin North America, says that the company's next-generation Energy Saver tire can cut rolling resistance by a further 20 percent over its current line of low-rolling-resistance tires, without shortening tread life or reducing grip. Michelin expects that a 50-percent reduction in resistance is possible in the next 10 to 15 years.

Does Rolling Resistance Change As Tires Get Older?

There are significant changes in the rolling resistance characteristics of a tire during break-in and initial operation. Tread rubber changes permanently during the first 4,000 miles of use, resulting in lower rolling resistance. Tire rolling resistance typically diminishes by about 20% throughout the life of a tire as the tread wears from full depth to worn out. This can be attributed to the reduction in tread mass and rubber squirm, as well as subtle hardening of the tread compound during years of service and exposure to the elements.

While this gradual reduction in tire rolling resistance and minor increase in fuel economy may be too subtle to register during the tire’s life on a month-to-month basis, the switch from well worn tires to new tires (even if they are the same brand, type and size) will result in an increase in rolling resistance of about 20% and the driver should expect to experience a potential 2% to 4% decrease in fuel economy. A tire with thicker tread will have higher rolling resistance only until the added tread wears down to the tread depth of the thinner-treaded tire.

Making Your Choice for Replacement Tires

A tire's inflation pressure, weight, and rubber chemistry all contribute to how much fuel it takes to push that tire down the highway. Automakers often specify low-rolling-resistance tires as original equipment to enhance a car’s performance in government fuel-economy and emissions tests. When it comes time to replace the original equipment (OE) tires, it’s important to consider the full spectrum of tire performance and resource longevity. You might be searching for something with a longer tread life, or a wider margin of safety should the weather turn unexpectedly wet or snowy, but if you are happy with your OE tires, you might just want to replace them with the same model.

Today’s tires are required to have ratings printed on the sidewall for traction, tread life, and temperature, but there are currently no standards for rolling resistance, and no data are kept. Rolling resistance varies with tire wear, condition, alignment, load, speed, inflation pressure, and road texture, so exact calculations are difficult to come by, but a new global testing standard (ISO 28580) should help.

When standards are in place, and rolling resistance ratings are available to consumers, it will be possible to weigh rolling resistance with other factors like tread wear, load and traction ratings to make an informed choice.

What’s the Prognosis for Rolling Resistance Standards?

Both California and the federal government have passed laws to label tires by fuel efficiency, but progress is slow. Even as the $34-billion tire industry spends huge sums trying to improve the efficiency of its products, tire makers oppose setting minimum standards and testing their products.

The state of California is working on rules and regulations for testing and rating tire rolling resistance, to be implemented in spring/summer 2009. It will be a while before rolling resistance ratings are universally available in the marketplace. Meanwhile, the most popular replacement tires marketed in the state will be the first to be tested, and the California Energy Commission will post information on their website (see below) as soon as it is available. Eventually, minimum standards for replacement tires will be put into place.

At this time, Consumer Reports (CR) is the only independent source providing consumers with comparisons of tire rolling resistance. CR measures rolling resistance through a protocol known as the SAE J1269 test for steady-state rolling resistance measurement. The ratings become part of the weighted average of all the performance tests that make up the overall score for tire model. In testing aftermarket all-weather tires, CR has found that it's possible to get a very good performing tire with low-rolling resistance compared to other tires.

Overall, Michelin figures that tires account for about 3 to 5 percent of the nation's CO2 emissions and about 9 percent of U.S. oil consumption. Our country burns nearly 4 billion barrels of gasoline and diesel fuel per year for transportation. Doing the basic math, 3 percent of 4 billion barrels is about 120 million barrels a year. That's a lot of fuel going to turning tires.

There are several ways to cut the amount of resource consumption related to your tires. You can reduce your vehicle’s fuels usage by lowering tire rolling resistance. You can cut resource usage by buying more durable tires, and discarding fewer of them. But, in the end, the best way to reduce resource usage is to eliminate driving mileage by bicycling and walking whenever possible, combining trips, carpooling, tele-commuting, and using public transit.

Web Resources:

www.energy.ca.gov/tires/index.html

www.consumerreports.org

www.tirerack.com

www.michelin-green-meter.com