DETROIT – In a normal year, the 2009 Chevrolet Cobalt wouldn't be any different than the 2008 model, save for a few cosmetic changes. But this is far from a normal year.

With gasoline still hovering between $3 and $4 per gallon, many manufacturers are making far more than the usual tweaks to cars and trucks between model years to squeeze out an additional one or two miles per gallon to attract customers who increasingly rank fuel economy as a top factor when buying a vehicle.

Bloomberg News

Brandon Depp works on a Chevrolet Cobalt at a plant in Lordstown, Ohio. GM tweaked the Cobalt to get an extra mile per gallon on the highway.

Automakers say you can expect more of the same as they roll out new technology without waiting for full vehicle updates.

"Fuel economy is very important," said Greg Peterson, General Motors Corp.'s vehicle performance manager for compact cars.

In the high-mileage version of the Cobalt and its Pontiac sister, the G5, GM engineers varied the intake and exhaust valve timing to make the 2.2-liter four-cylinder engine burn fuel more efficiently.

They arranged with Goodyear Tire & Rubber Co. for tires with lower rolling resistance, and they changed the gear ratios of the five-speed manual transmission so the engine revs more slowly at highway speeds.

The result: an extra mile per gallon on the highway, boosting the Cobalt and G5 XFE models to an Environmental Protection Agency estimated 37 mpg.

At Ford Motor Co., engineers added a six-speed automatic transmission, electric power steering and variable valve timing to the Escape and Mercury Mariner small sport utility vehicles to get another two miles per gallon on the highway.

Changes were made in other models to get similar improvements, a spokesman said.

Six-speed transmissions, which are used by most automakers, make vehicles more efficient. On the highway, they require fewer revolutions per minute, increasing efficiency. Electric power steering reduces drag on the engine by removing the belt that powered the old hydraulic system.

At Chrysler LLC, engineers took similar measures on several models but also recalibrated gas pedals, changed to more efficient air conditioning compressors and tweaked transmission shift intervals to make them more efficient.

For instance, Chrysler was able to push the highway mileage of the Sebring and Avenger midsize sedans to 31 mpg, up from 30.

"These are things that we can do right now for the customer," said spokeswoman Sue Keighron. "They may have been changes that we would have made, but not necessarily as quickly as we are doing now."

Honda Motor Co. and Toyota Motor Corp., which have led automakers in fuel economy in recent years, each said they weren't making similar changes to existing models, partly because they're already using some of the new technology.

"Honda's been a fuel economy leader for an entire generation because we bake in good fuel economy at the design stage," said spokesman Ed Miller.

The company does make changes to existing vehicles between model years when technologies are developed, Mr. Miller said.

The Odyssey minivan V-6 engine, for example, was given the ability to work on three, four or six cylinders for the 2008 model year, a spokesman said.

With the U.S. auto market continuing its shift from trucks and SUVs to more efficient cars and car-based crossovers, automakers say they'll keep adding technology to make their cars more efficient.

When new models come out, look for more dramatic improvements. GM, for instance, says the Cruze, the Cobalt's replacement coming in the second half of 2010, will get around 45 mpg on the highway.

Although the gains may seem small between model years, they will add up, the automakers say.