A formal evaluation of its new car mileage estimates by the Environmental Protection Agency found that what a lot of drivers have been saying is right: Those miles per gallon numbers aren't much good.

The agency yesterday made public a 26-page report involving several separate studies of its estimates at the regular bi-monthly meeting of automobile manufacturers at the EPA laboratory here.

Four key conclusions from the report are:

Fuel economy for 1975 and 1976 model cars in everyday use is approximately one mile per gallon less than the EPA city number and four mpg less than the highway rating.

The difference is greater for smaller cars, with sub-compacts getting 7 mpg less than the EPA highway estimate.

By country of origin, German-made cars equaled or excelled EPA estimates, Japanese cars lost the most mileage, and domestic cars fell in between.

Most ominously, the rankings in the EPA mileage guide aren't very accurate. A 2-to-5-mile-a-gallon difference is necessary to be even modestly sure that two individual cars are in the right order.

John De Kamy, EPA director of emision control technology, said some of the discrepancies result from differences between the prototype cars the agency tests for air pollution and the production automobiles that are built later.