In an attempt to make "customer expectations more realistic," the Environmental Protection Agency is going to issue only one mileage rating for each type of new car product instead of the present three-rating system.

EPA Administrator Douglas Costle said that beginning in 1979 the agency will list only the expected miles per gallon for city driving, dropping expected highway mileage and a figure combining the two.

In announcing the decision, Costle said, "The single number will be called the estimated MPG. It now comes the closest of the three existing ratings to matching the miles per gallon that most drivers are getting."

Costle said that consumer complaints "clearly showed the highway and combined ratings to be unrealistically high".

Calling the move to using only the city figure "a temporary step," Costle said, "It's the most immediate action we can take to make the 1979 ratings more accurately reflect consumer experience."

There will be further changes in the testing procedure which should help the agency to gather more accurate data, Costle added.

"Dropping the highway and combined ratings removes that irresistable but human urge to accept the highest numbers available in buying a car," Costle said.

he figures are published annually in a "mileage guide" distributed to the public and to dealer showrooms. The figures also appear on window stickers on all new cars.

Three of the four auto companies said the announced change would make the rating system less confusing, but just as disappointing for car buyers expecting to reachthe published figures.

"We felt a range of numbers would be more satisfactory than to suggest to people that they are going to get a precise mileage figure," said a General Motors Corp. spokesman.

A Ford Motor Co. spokesman said he was worried that the single number would give the impression that the driver should achieve that level of performance.

"People see it and they think they are going to get that kind of mileage," he said. "What it was initially intended to do was provide a means of comparing one car to another."

A Chrysler Corp. spokesman said the single number "probably will be less confusing," but said his firm preferred using the combined mileage figure.

But George Brown, American Motors Corp's director of vehicle emissions and safety, supported the change, calling it "a lot closer to the real would experience of most people."

Brown said that the makers of some of the foreign imports, which have five-speed transmissions, will be hurt by the change because they achieve particularly high fuel economy on the highway.