WASHINGTON -The energy bill President Bush signed last week mandating tougher fuel-economy standards sent a simple message to automakers: lighten up.

The new rules certainly give makers of aluminum, carbon fiber and other lightweight materials something to smile about, analysts say, while the steel industry's piece of the auto-industry pie is likely to shrink.

Auto shoppers, meanwhile, can expect to pay a premium at dealerships when the new rules kick in - but the effect will be mitigated somewhat by fuel savings.

The new law requires each manufacturer to raise its fuel-economy average for its overall fleet 40 percent in the United States, to 35 miles per gallon, by 2020. Increased mileage requirements could begin as early as 2011.

"With new standards, historically the auto industry has responded by lowering the weight, which meant less steel and more aluminum, rubber and plastic," said Mary Deily, a professor of economics at Lehigh University in Bethlehem, Pa., who has studied the steel industry.

A 10 percent drop in weight yields roughly a 6 percent improvement in fuel economy, automakers and analysts said.

To fully achieve the energy bill's fuel-economy goals, however, automakers also are looking at enhanced engine and transmission efficiency (already found in gas-electric hybrid vehicles), reduced tire resistance, and improved aerodynamics, said Alan Taub, executive director of research and development at General Motors Corp.

"The question is how to deliver this fuel economy with the best combination of technologies to deliver the highest value to customers," Taub said.

Today, steel accounts for about 60 percent of an average vehicle's weight in the United States. That is less than a generation ago, when much more steel was used. Still, the popularity of heavier-weight trucks, minivans and SUVs has caused the average vehicle weight to rise more than 25 percent, to about 4,100 pounds, over the last 20 years, helping steel providers.

Even so, the percentage of aluminum in cars has been on the rise for decades since the last boost in fuel-economy standards. Alcoa Inc., the country's largest maker of aluminum, sees an even greater opportunity ahead.

Hexcel Corp., Zoltek Cos. Inc., and other carbon-fiber makers also stand to benefit from tougher fuel-economy rules. Their lightweight composite materials, which are significantly more expensive than steel, already are used in some Mercedes, BMW and Audi vehicles and in GM's new Corvette, as well as in the aerospace industry, which is looking to drive down its jet-fuel expenditures.

As aluminum and carbon fiber replace some steel, there will be a "fairly serious cost impact" for consumers, said Larry Rinek, a senior automotive consultant with Texas research and consulting firm Frost & Sullivan Inc.

Still, Alcoa spokesman Kevin Lowery said aluminum costs would drop over time as automakers, to reduce waste, get Alcoa and other metal producers involved earlier in the production process. Zoltek, Hexcel and other carbon-fiber makers already are ramping up production to meet an anticipated surge in demand. But the mainstreaming of carbon fiber as a car-building material depends on prices coming down, and that can occur only with mass production, said Brian Yerger, an alternative energy analyst at Jesup & Lamont Securities.

While Europe favors small cars, manual transmissions and diesel engines to offset high fuel prices, U.S. automakers believe they can make cars lighter, and more energy-efficient, without sacrificing size.

For example, Ford Motor Co. last month said that state-of-the-art engines and power-steering systems would help it meet a portion of the fuel-efficiency mandates and that greater use of aluminum and high-strength steel could help shed 250 pounds to 750 pounds per vehicle.