Automakers are working continuously to boost the fuel efficiency of sedans

(The Associated Press)

Customers shopping for midsize cars generally have families and need space for children, yet they are budget-conscious. So to improve gas mileage, automakers are tweaking cars and trucks between model years -- especially in their midsize sedans.

Honda, for example, recently announced changes in its Accord sedan that boosted highway mileage by almost 10 percent from the 2010 to 2011 model years. Honda says engineers improved the Accord's aerodynamics, reduced engine friction and changed the top gear in the automatic transmission to boost highway mileage to 34 mpg from 31 mpg and city mileage to 23 mpg from 21 mpg.

Honda boosted its 2011 Accord highway mileage by almost 10 percent. (Honda)

The Accord, the second-best-selling car in the U.S., moved from near the bottom of the midsize pack to second place in gasoline-powered highway mileage. It was bested only by the Hyundai Sonata, at 35 mpg.

The moves come as automakers increasingly use advanced technology to raise fuel economy with their internal-combustion engines. Many have made four-cylinder engines as powerful as older six-cylinder motors. They're using transmissions with six or more speeds, direct fuel injection and tires with less resistance.

Honda changed the Accord's transmission so the four-cylinder motor doesn't work as hard at highway speeds. It used thinner oil to decrease friction in internal parts, and it changed the floor panels to make air flow more smoothly beneath the car, says spokesman Jon Fitzsimmons. It also reduced the rolling resistance on the tires.

Hyundai also has seen the need to better the miles per gallon in its midsize cars.

"For the folks who are staying in sedans, they tend to put a real premium on fuel efficiency," says John Krafcik, chief executive officer of Hyundai Motor America.

Hyundai executives decided early on not to offer the 2011 Sonata with a six-cylinder engine, saving 50 to 100 pounds of weight in a move that helped boost fuel economy, Krafcik says.

The company instead designed a four-cylinder engine that directly injects fuel into the cylinders surrounding the pistons. It uses less fuel and generates more horsepower than conventional engines. Other automakers are either using the technology or working on it.

Chrysler, which is at the bottom of the midsize pack in fuel economy with its Dodge Avenger and Chrysler Sebring models, is in the midst of improving them for the 2011 model year. It promises significant improvement in gas mileage, although the company will not give numbers yet.

The reworked Sebring and Avenger are due out in the fourth quarter of this year. They now get 21 mpg in the city and 30 on the highway when equipped with a four-cylinder engine and automatic transmission.