General Motors Corp. unveiled a new family of fuel-efficient three-, four-, five- and six-cylinder engines yesterday and President E.M. Estes promised that GM's cars will average 31 miles per gallon by 1985.

Estes said the nation's biggest car maker plans to increase its production of high-mileage cars so rapidly that within five years GM's fleet will get 3.5 miles per gallon more than required by federal regulations.

Because of soaring gasoline prices, "the market is way ahead of the government's fuel-company standards," the GM president said, insistisng there is no need for tightening federal mileage standards.

General Motors' contention that the public wants -- and the car makers will make -- cars that get better mileage than the government requires, was meant to head off efforts to boost the minimum mileage required by law. Conservation advocates like Sen. Henry Jackson (D-Wash.) went to push the federal mileage standards up to 50 miles to the gallon.

A 49-mpg "commuter car" with an experiimental three-cylinder engine was among the surprises shown to reporters at the General Motors Technical Center in Detroit yesterday morning. After rolling out a showroom full of miserly motorcars, Estes flew to Washington where he repeated, at a lunch with local auto dealers, his pledge to make gas guzzlers extinct.

Until yesterday the three-cylinder mini-car was only a rumor in Detroit and even the less conventional V-5 was unheard of GM engineers said the engines are part of a new family of engines using 85 percent of the same parts that can be increased in size just by adding another cylinder.

The company showed an in-line three, a V-4, a V-5 (made by mating the three with two cylinders from the V-4) and a V-6 (two threes put together). Although it displaces only 180 cubic inches (3 litres), the V-6 is powerful enough to handle the a six-passenger car, engineers said.

Also shown for the first time was an experimental gas turbine that General Motors officials said can be made to run on anything from kerosene to gasahol to powerful coal.

That engine is a long way from production, the company admitted, but Estes told Washington dealers they weill be selling electric cars by 1984 if the project can be kept on schedule. The GM electric, shown in Detroit, is a two-seater with room in the back for groceries or golf clubs.

General Motors' plan is to offer a high-mileage option -- with a diesel or a four-cylinder engine and a five-speed transmission -- on all its models, Estes said. "We're going to try to take fuel economy out of the buying decision," he said, nothing that "95 percent of our cars have the potential to exceed 30 mpg in 1985."

Estes said the Carter-administration's auto-aid package announced in Detroit yesterday in the first step toward creating the partnership of industry, government and labor that already exists in Germany and Japan. "There may be nothing specific," he added, "but there is a tremendous change in attitude."