DETROIT — General Motors Corp. is putting a six-speed automatic transmission mated with a four-cylinder engine in the 2008 Chevy Malibu, giving the automaker some fuel-efficiency bragging rights in the hyper-competitive midsize car segment.

The new power train debuts immediately in the high-end Malibu LTZ and next year in two lower-priced Malibu models. The LTZ's fuel economy, as estimated by the U.S. Environmental Protection Agency, is 32 miles per gallon on the highway — up from 30 mpg on the existing four-cylinder Malibu with a four-speed transmission.

The equivalent four-cylinder Toyota Camry and Honda Accord models offer 31 mpg.

"We're the only guys in the segment with a six-speed transmission," Dave Whittaker, Chevrolet's vehicle line executive, said yesterday at a media briefing. He added that with record high oil prices, "all consumers are very sensitive to fuel economy."

GM said the efficiency boost of the six-speed also offers a quieter ride, slight acceleration improvement and smoother shifting because the revolutions-per-minute drop is not as great.

The 2008 Malibu has been popular, with sales up 31 percent year-to-date to 41,247 cars. It ranked 18th on Autodata Corp.'s list of the 20 top-selling vehicles in the U.S. for March. The Camry ranked third and the Accord fourth.

Six-speed automatic transmissions will spread to more cars and trucks as automakers work to make vehicles more efficient, said Kevin Riddell, power train forecasting manager for J.D. Power and Associates.

Other manufacturers, including Ford Motor Co., Toyota Motor Corp. and Chrysler LLC, already have six-speeds in place, while others use more-expensive continuously variable transmissions that don't shift like conventional gearboxes, Riddell said. Mercedes-Benz even offers seven speeds, he said.

Six-speeds can keep an engine revving at its peak efficiency better than four- and five-speed transmissions, Riddell said. Manual transmissions, he said, still are more efficient, although six-speed technology brings automatics closer.

Ford has no four-cylinder models with six-speed transmissions, but it plans to introduce the new power train in the 2009 Ford Escape in August, spokesman Jim Cain said. It would boost fuel economy in the small sport utility vehicle by 1 mpg to 27 on the highway, he said.

Ford has had six-speeds for several years in models such as the Edge crossover and Fusion V-6 sedan, and it expects wider use of the transmissions.

"You see a tremendous benefit in terms of overall drivability and highway fuel economy," Cain said.

Honda Motor Co. spokesman Edward Miller said the Accord offers the best combination of horsepower, interior space and fuel economy with either the four- or six-cylinder. For 1 mpg less than the Malibu LTZ, the Accord offers 177 horsepower and an interior that qualified it for the EPA's large-car category.

He said he cannot discuss Honda's plans, but "we've met all of our targets with the five-speed automatic transmission."

Bruce Belzowski, assistant research scientist with the University of Michigan's Transportation Research Institute, said the six-speed transmissions are part of automakers' strategy to make the internal combustion engine as efficient as possible while they develop alternative power sources.

"Right now, the strategy is try to eke out as much as you can out of the internal combustion engine before you have to go into more exotic power trains ... whatever flavor of hybrid you want to talk about," he said.

He said it's impressive that GM can get 32 miles per gallon out of a midsize car, and it's a sign that automakers are working to comply with new federal fuel economy mandates.

On the same day as GM's Malibu announcement, the Bush administration revealed proposed rules that would require new cars and trucks to meet a fleet average of 31.6 mpg by 2015, a schedule more aggressive than initially expected by the auto industry. A new energy law requires new cars and trucks, taken as a collective average, to meet 35 mpg by 2020.

A 1-mpg difference between competitors may not sway buyers toward Chevrolet, but it's an example of how automakers are getting competitive on fuel economy, Belzowski said.

"One mile per gallon, it's not insignificant, but to say 'we're the leader' kind of implies that you're so far ahead of everybody else. It's a marketing gimmick that only takes you so far," he said.

Still, Whittaker said, any advantage is important in one of the industry's biggest segments.

"The goal ... is not just to be competitive or as good as," he said. "The goal is to win, to be the best."