For the second consequtive year, the Volkswagen Rabbit diesel has been designated the most fuel-efficient car by the Environmental Protection Agency, averaging 41 miles per gallon.

EPA's annual report card on automobile fuel efficiency shows that none of the top 10 most efficient cars is manufactured in the United States. However, several of the cars, such as Dodge Colt and the Plymouth Champ, carry the nameplates of U.S. companies.

Volkswagen held the top three spots with Rabbit and Dasher diesel modes, followed by the Datsum B-210 and six different Plymouth and Dodge models that are built predominantly in Japan.

Not unexpectedly, large U.S. cars such as Cadillac, and expensive and ornate foreign cars such as Jaguar guzzlers, getting as few as 7.6 miles and Aston Martin, were the worst gas per gallon.

The average for all 1979 models tested thus far (some automakers, such as Honda and Fiat, have not yet introduced their 1979 cars, which will be evaluated later) is 19.9 mpg, up slightly from last year's 19.6.

The Energy Policy and Conservation Act requires that each auto manufacturer average 19 mpg. for its entire line at the end of the sales year. This is 1 mpg. higher than last year.

EPA's figures for 1979 model-year cars reflect a new, one-number system instead of the highway and city driving breakdowns of previous years.

EPA Administrator Douglas M. Costle said the single number is "roughly equivalent to last year's city driving number." He said the agency decided to use the single, lower figure because "our tests show that most people, in day-to-day driging situations, achieve fuel company ratings closest to this number."

But despite the fact that EPA will be publishing only the city driving number, and that number will be the only one to appear on the mileage sticker required on each new car, the agency says it will keep records of highway mileage tests for each car.

A Federal Trade Commission official said there is currently no reason why auto companies cannot use the highway mileage figures in ads promoting specific cars.

"We are consideraing whether we should ban or restrict in some wat the use of the highway numbers," said FTC energy and product information official Ann Nachbar. "But until we make our decision the manufacturers can use the highway figures."

The EPA also calculated estimated annual fuel costs for every car, using an average of 70 cents per gallon of gasoline, or 60 cents per gallon of diesel fuel, and an average use of 26,000 miles per yard.

The top-rated German-made Volkswagen was estimated at $220 a year in driving costs, while the worst gas guzzler, the 7.6 mpg Aston Martin V8, has an estimated annual cost of $1,312.

The top 10 1979 car models in terms of efficiency:

1. VW Rabbit diesel with 90 cubic inch engine and 4-speed manual transmission - 40 mpg.

3. VW Dasher diesel, 90 cubic inch engine, 4-speed manual transmission - 36 mpg.

4.Datsun 210, 85 cubic inch engine, 5-speed manual transmission - 35 mpg.

5. Dodge Colt hatchback, 86 cubic inch engine, 4-speed manual transmission - 34 mpg.

6. Plymouth Champ, 86 cubic inch engine, 4-speed manual transmission - 34 mpg.

7. Dodge Colt hatchback, 98 cubic inch engine, 4-speed manual dualrange transmission - 33 mpg.

8. Plymouth Champ, 98 cubic inch engine, 4-speed manual transmission - 33 mpg.

9. Dodge Colt hatchback, 88 cubic inch engine, four speed manual dual range transmission - 32 mpg.

10. Plymouth Champ, 86 cubic inch engine, 4-speed manual dual-range transmission - 32 mpg.

The U.S.-made car finishing with the best mileage rating was the 4-speed manual, 98 cubic inch. Chevrolet Chevette, which averaged 29 mpg.

Costle stressed that the mileage figures were only estimates derived from treadmill tests, not on-the-road driving, and actual figures depend on the individual driver and such variables as weather and road conditions.

"These numbers are ment to be used only as comparison figures," he said.

Costle also announced yesterday that the EPA will continue to permit some 65 gas stations in Nebrasko Illinois and Iowa to market gasohol - an experimental mixture of gasoline and alcohol - while pollution and mileage studies continue on the controversial product.