American cars have made their strongest showing since the government began testing for fuel economy in 1973 as three 1981 models -- the Chevrolet Chevette, Ford Escort and Lincoln-Mercury Lynx -- registered 30 miles per gallon in simulated city driving, the Environmental Protection Agency said yesterday.

Although the Volkswagen Rabbit diesel topped the study at 42 mpg, leading the field for the fourth consecutive year, the results showed significant gains for American cars. The highest-rated car last year was listed at 26 mpg.

With the nation's competitive position in the auto industry a subject of continuing debate, EPA Administrator Douglas Costle hailed the result.

"The American auto industry in 1981 is offering a wide range of small, fuel-efficient cars that should help their competitive position, as well as national fuel conservation goals," Costel said.

Still, the top 10 cars in the EPA study were imports, just as they were last year. Following the Rabbit diesel in the results were the Toyota Starlet at 39 mpg, a Volkswagen gasoline-powered Rabbit at 38 mpg and a Japenese-made Dodge Colt at 37 mpg.

Rounding out the top 10 in the EPA study were the Plymouth Champ at 37 mpg, the Datsun 210 and Toyota Corolla Tercel at 36 mpg and the Honda Civic and two models of the Mazda GLC at 35 mpg. All of these cars, including the Champ, are manufactured in Japan.

Though the mileage figures, based on simulated city driving, must be posted on new cars, the EPA warned that they do not reflect road and weather conditions or individual driving habits. Thus the mileage these cars get in real driving may be different.

Until two years ago, EPA published both a city and a highway mileage figure. But that system was dropped in light of complaints about the reliability of the highway figures.

Federal fuel efficiency regulations require 1981 model cars to average 22 mpg for each of the major manufacturer's divisions. In four years, that figure is to rise to 27.5 mpg.

Following close behind the three domestic leaders were the Dodge Omni, the Plymouth Horizon, a slightly larger Ford Escort and another Lincoln-Mercury Lynx, all of which were tested at 28 mpg.

A Dodge 024 De Tomaso and a Plymouth TC3 Turismo were listed by the EPA at 27 mpg.

The highest rated mid-size cars are the Dodge Aries and the Plymouth Reliant, which both were tested at 25 mpg.

The lowest ratings of any of the cars tested, a sample that includes just over 80 percent of the 1981 model cars, were reported for the Chevrolet Corvette, a Pontiac Firebird and three models of Cadillacs. Those five cars all were listed at 15 mpg.

The top-rated Volkswagen Rabbit diesel, a four-speed manual transmission model, is assembled at Westmoreland, Pa., along with all other Rabbits sold in this country. But the Rabbit is considered an import because most of the parts and a considerable amount of the labor that goes into the car's production comes from abroad.