The engineers who create gallon-squeezing cars like the Toyota Prius use every available method to comply with the ever-tightening fuel-economy standards they must meet. Those solutions may include the use of hybrid powertrains, adopting lightweight materials, cutting aerodynamic drag and minimizing the friction losses of moving parts.
There's another path to fuel efficiency that may be less attention-getting, yet rather effective: equipping the cars with low-rolling-resistance tires.
Such tires can improve fuel economy by as much as 1 or 2 miles per gallon over conventional all-season designs — and more when compared with high-performance summer tires — according to automakers, making them an important tool of the pursuit of vehicle efficiency.
Given the high cost of developing engine technologies such as turbocharging and direct fuel injection, working to improve tires can deliver worthwhile improvements for relatively small investments. The negative side of easy-rolling tires has been a lingering reputation, based on early examples, for compromised traction in wet conditions and, in some cases, short tread life.
Tire companies say that advances in the materials used in the latest low-rolling-resistance tires, and in the manufacturing processes, have overcome the drawbacks.
"There have been significant trade-offs with this type of tire, namely wear performance and stopping distance," says Sheldon Brown, executive program manager at the Toyota Technical Center in Saline, Mich. "As tire manufacturers continue to tune their technology, we have seen improvements in the compound technology to address wear."
The good news for drivers is that tire manufacturers are starting to build the fuel-saving features into all tires.
Forrest Patterson, technical director for North American passenger car and light truck tires at Michelin, says the Ford Fusion Hybrid provided a good example of what the carmakers want in a tire design. "Ford really worked with all their suppliers to deliver a vehicle with 40-plus miles per gallon, and our tires were a big part of that," he says.
On average, 5 to 15 percent of the fuel a car burns is used to overcome rolling resistance in the tires, according to the Department of Energy. Rolling resistance is generated largely in the tire's outer tread band, which deforms as it comes into contact with the pavement, then springs back.
The resistance caused by this flexing of the tread generates heat in the rubber, wasting energy without providing any benefit. The tire designer's challenge is to let the tire flex enough for good contact with the road while minimizing internal friction and wear.
Over the past 20 years or so, tire makers have begun using compounds with names such as solution styrene-polybutadiene rubber and neodymium polybutadiene rubber as ingredients for meeting their performance goals. Along with silica, these materials have reduced the rolling resistance of tires by 25 to 30 percent, according to Lanxess, a German synthetic-rubber and specialty-chemical supplier.
As a marketing exercise, Lanxess recently demonstrated a prototype tire called the AA Concept to showcase the performance of its advanced formulations. The AA nomenclature refers to Europe's new labeling system, which ranks tires from A to G on wet grip and fuel efficiency, with A being the highest ranking.
In the United States, the National Highway Traffic Safety Administration is developing updated tire labeling rules that will add grades for fuel efficiency to help consumers in selecting replacement tires.
Goodyear, which supplies low-rolling-resistance tires as original equipment on the Toyota Prius, Chevy Volt and other high-efficiency cars, says its Assurance Fuel Max tires have improved traction and wear life — and they brought a 27 percent reduction in rolling resistance compared with the company's previous generation of Assurance tires. That translates into 4 percent better fuel economy than the earlier generation of tires, Goodyear says.