DETROIT -

Modern car designs at NAIAS are being shaped by government fuel rules.

Automakers are experimenting with new materials and thinner, more aerodynamic shaping. The focus is on smaller, lighter vehicles.

The tough new fuel economy standards require manufacturers to achieve a fleet average of 54.5 miles per gallon by 2025.

Most car companies have already worked through the easy approaches, such as putting smaller engines into bigger cars.
Ford's most recent step is switching to an all-aluminum body for its best-selling vehicle, the F-150. The switch from steel has taken up to 700 pounds out of the vehicle, making fuel-economy numbers sure to go up. Most other automakers are following suit, and using more aluminum.

Ford is also banking on its EcoBoost engine, which gets more power out of smaller motors.

Meanwhile, Chrysler is switching to a nine-speed automatic transmission in its latest vehicle, the 2015 Chrysler 200. More gears are better able to keep engines operating at their most efficient. The Chrysler 200 now boasts 35 miles of highway fuel economy.

General Motors is making more midsize trucks, like the Chevrolet Colorado and GMC Canyon. Those trucks are now offered with four-cylinder motors in an effort to be more fuel-efficient. In 2016, GM engineers say they will add a turbo-diesel option for trucks.

Toyota is sticking with more hybrids. The Japanese automaker is expected to bring a hydrogen-powered fuel cell vehicle to the market soon.