Low-Rolling Resistance Tires Can Help You Get Better MPG

The National Highway Traffic Safety Administration is proposing a new sticker for tires to provide consumers more information including a wet traction index and tread wear rating. The sticker would also include a fuel efficiency rating based on the tire's rolling resistance, a nod to increasing buyer interest in environmental performance.

Tires can have a significant effect on fuel economy. From five to 15 percent of the fuel used by a typical car goes into overcoming rolling resistance. Energy is wasted as heat, which is produced by the friction between road and tire, by the flexing of tire walls, and between the rim and tire. Touch a tire after a ‘hot run’ and you’ll appreciate how much heat is generated and just how much energy is wasted.

Virtually every tire manufacturer is developing, and indeed many are already marketing, low rolling resistant (LRR) tires that minimize wasted energy. The payoff can be quite significant. For instance, a 20 to 25 percent reduction in rolling resistance can increase fuel economy by three to five percent. Almost every hybrid vehicle is fitted with low rolling resistance tires. They are even more critical for pure electric vehicles where reduced rolling resistance translates into increased range. For example, Michelin developed super low rolling resistance tires specifically for GM’s EV1.

Low rolling resistance can be achieved by using advanced rubber compounds with more efficient tread patterns. There is a tradeoff between optimized rolling resistance and grip resulting from tread design. Intricate tread designs provide better grip, but the tread traps air in the pockets when in contact with the road. The tire has to compress air, water, dirt, and snow trapped in these pockets, requiring the engine do more work and use more fuel.

A tire’s rolling resistance coefficient (RRC) is rolling resistance force divided by wheel load. The lower the RRC, the less energy required to overcome rolling resistance. Most tires on new passenger cars have RRCs ranging from 0.007 to 0.014.

Tire life, traction, and temperature resistance can be as good for low rolling resistance tires as conventional tires. About the only downside is that they are bit harder. Thus, some drivers may feel that tires with lower rolling resistance don’t hold the road as well. Aggressive drivers might want to use a softer tire for better road adhesion. Hypermilers will go for the lowest resistance tire found on the market to help them achieve the highest fuel efficiency possible. Because of the more sophisticated materials used, low rolling resistance tires can be a bit more expensive, but the extra cost is recouped through fuel savings.

Keep in mind that the advantages of low rolling resistance tires can be negated by improper tire inflation. For example, rolling resistance can increase by up to 12 percent if a tire is inflated to 28 psi rather than a required 35 psi. Plus, for every 10 degrees F change in air temperature, tire pressure will increase (with higher temperatures) and decrease (with lower temperatures) by about one psi. Improper wheel alignment can also increase rolling resistance. When the time comes to replace tires on a hybrid, buy the stock tire that’s designed to be used with a particular hybrid model. Some owners have experienced as much as a five mpg penalty by doing otherwise.

Replacement tires for conventional vehicles generally have great all-season grip and longer tread life, but often not optimal rolling resistance. U.S. auto manufacturers typically equip new vehicles with tires that have lower rolling resistance than the average aftermarket replacements to help them meet Corporate Average Fuel Economy (CAFE) requirements.

The new stickers mentioned earlier are actually a requirement of the Energy Independence and Security Act of 2007, with a final rule to be issued by December 2009. The new three-tiered ratings also will appear on www.safercar.gov. Currently, Consumer Reports publishes a rolling resistance score as part of its tire ratings.

Finally, as in many things green, California is leading the way with the world's first fuel-efficient tires law. It will require replacement tires that are of the same quality, performance, and efficiency as new car tires. The law also requires the development of low rolling resistance standards and printing appropriate ratings on tire sidewalls. All tires that rank within 15 percent of the lowest combined tire size designation and load rating will be designated a fuel efficient tire.