One way to make a car go farther on a gallon of gas is to make it lighter.

And one way to reduce a car's weight without making it smaller is to replace steel parts with lighter ones made out of other metals, such as aluminum and magnesium.That's why U.S. automakers announced two initiatives Thursday to accelerate the development of lightweight metal parts that don't make cars more expensive or less safe.

Ford Motor Co. said it has begun testing 20 Mercury Sables with all-aluminum bodies, and that another 20 will be built and shipped to fleet users worldwide to see how they hold up and whether consumers like them.

"Placing these cars in the hands of real-world users . . . is a decisive step in what we see as a promising technology," said Kenneth Kohrs, the Ford vice president in charge of car product development.

The aluminum Sables look exactly like the ones made with steel doors, fenders, roofs and underbodies, but weigh 400 pounds less.

Ford, General Motors Corp. and Chrysler Corp. also announced creation of a joint research effort to make lightweight metal parts.

Although the average American car already has about 200 pounds of lightweight metal parts, replacing steel is not as easy as it might sound. Among the problems:

-It is easier to make and assemble steel parts than those of aluminum or other lighter substitutes.

-Lighter metals are often more expensive than steel.

-Lighter metals are not as strong as steel, raising safety concerns.

The Big Three also said they would work together on new, high-performance supercomputer systems and programs to speed the development of lightweight parts and other components.

It now takes supercomputers about 20 hours to simulate a car crash. George Dodd, a GM engineering manager, said the automakers hope to cut that to two hours.