FUEL ECONOMY FRENZY

A big fuel saver: Easy-rolling tires (but watch braking)


Jesse Snyder

Automotive News | July 21, 2008 - 12:01 am EST

As gasoline prices soar, auto and tire manufacturers are taking a fresh look at low-rolling-resistance tires.

Tire makers say automakers are asking for easier-rolling tires as a quick way to boost fuel efficiency.

"We have increased interest in anything that can improve fuel economy," said David Cowger, General Motors program engineering manager for tire development. "If we reduce rolling resistance by 10 percent, it can improve fuel economy 1.5 percent."

The trick is in the trade-offs. Automakers need tires to produce more mileage without making major sacrifices in stopping distance and handling, ride quality or durability.

Normally, better rolling resistance equals less traction. Tire makers say they are developing compounds that break that equation, but they apparently add cost.

Yet with fuel economy on the minds of consumers who are suddenly paying more than $4 a gallon for gasoline, marketers are touting low-rolling-resistance tires on vehicles such as the Chevrolet Cobalt and 2009 Ford Escape and Mercury Mariner.

In April, Michelin broke new advertising emphasizing the fuel savings of its Energy replacement tires. In June, tire maker Hankook quadrupled r&d spending on fuel-saving tires to $19 million annually.




How they work

Easier-rolling tires absorb less engine energy to move, improving fuel economy. Most other mpg-boosting tricks - lower vehicle weight, better aerodynamics, more efficient powertrains - take years to carry out. So changing tires is an obvious response to a rapidly changing marketplace.

Rolling resistance is a measure of how much propulsion energy is lost by tires flexing as they roll across the road surface. Most resistance comes from the compression of the tread against the road and the flexing of the body of the tire as it rotates.

Making tires with low rolling resistance starts by reducing mass, typically by using thinner sidewalls and less tread depth. Next is adjusting the compound. Most tire compounds mix natural or synthetic rubber, which provides the grip, with reinforcing fillers such as carbon black, which adds stiffness and strength.

Braking trade-off

Low-rolling-resistance tires typically use more fillers and less rubber than high-performance tires that maximize traction.

The interplay of tires and other vehicle systems is illustrated in the Ford Escape crossover. Ford introduced a new-generation Escape in the 2002 model year and has made a series of changes to improve fuel economy.

Consumer Reports magazine documented the increased fuel efficiency, from 17 mpg in 2001 to 18 mpg in 2004 and 19 mpg in 2007 in tests of identically equipped Escapes. But it blasted Ford for a corresponding increase in 60-mph-to-0 stopping distances of the 2008 model. In December 2007, the magazine said the Escape's wet stopping distance rose from 146 feet in 2001 to 165 feet in 2004 and 215 feet in 2007.

"It's ironic that while the fuel economy improved in the same vehicle over successive tests, stopping distances increased to an excessive length," said David Champion, senior director of the Consumer Reports automotive test center in Colchester, Conn.

Although Ford lowered the rolling resistance of Escape tires from 2001 to 2007, Ford said the Consumer Reports tests cannot isolate tire performance because the automaker changed other systems over six years. "Those three Escapes were not mechanically identical," said Ford spokesman Alan Hall. Ford did not dispute the Consumer Reports brake test results.

Champion said each vehicle tested was a four-wheel-drive Escape XLT with 3.0-liter V-6 and four-speed automatic and had the same weight and body shape.

"I can't categorically say it's all the tires, but nothing else changed," he said.

Ford's re-engineered 2009 Escape, which started production in June, has low-rolling-resistance tires - but from a different supplier, Michelin.

"We wanted to change the tires because we also revised the brakes, suspension" and electronic stability control, said Ron Razzano, Ford vehicle engineering manager for the Escape.

He said Ford responded to media criticism and customer input in the redesign, which will noticeably improve the new Escape's braking performance.

The wet stopping distance for the 2009 Escape with 16-inch tires is 160 to 170 feet, depending on road surface, a Ford spokesman says.

New materials

Low-rolling-resistance tires don't have to sacrifice stopping distance, said David Stafford, COO of Michelin Americas Research Co.

"We use higher-performance materials in the tread compound - more silica as reinforcing fillers - and advanced tread design features," he said.

But that costs more. A set of four 225/60R16 Michelin Energy low-rolling-resistance tires mounted and balanced retails for $640, compared with $540 for four Michelin Symmetry tires.

Automakers say new materials are helping designers build better tires.

"It's gradual but always better," said GM's Cowger. "Over 15 years, I have seen an across-the-board 15 percent improvement in rolling resistance and a 20 percent gain in wet traction."