There may be no six-passenger cars or full-sized station wagons by the mid-1980s if the government persists in forcing the auto makers to manufacture cars that travel an average of 27.5 miles per gallon by 1985, Ford Motor Co. claims.

In a statement to the National Highway Traffic Safety Administration, Ford contends that, while it once believed the 27.5 mpg figure was feasible for the 1985 cars, it now thinks meeting such a goal would be too costly.

Ford's remarks are an expression of concern to NHTSA, which must report to Congress on the progress of the fuel economy standard program by mid-January.

Ford said that in making decisions about cars it will produce five and six years from now, it has found that, "in both social and economic terms, the cost of increasing fuel economy is far higher than was originally envisioned."

"Going from 26.5 mpg to 27.5 mpg will cost billions, yet it will save the average motorist less than 20 gallons of gasoline per year," Ford said.

Industry sources say Ford is close to being in trouble with the 1979 fuel economy standards as applied to its entire fleet. Auto makers must have an average 19 mpg on their total fleet of cars sold.

Because of low sales of Ford's beleaguered Pinto, and a surprising increased demand for larger cars, Ford's average is dangerously close to dropping below the 19-mpg cutoff, according to industry sources, and the company could face fines if it does drop that much.