The era of the big American car is ending; 1979 probably will be the last year in which the nation's auto plants produce more big cars than little ones, say Department of Commerce economists.

For the 1980 model year, they project U.S. auto output - totalling 8.2 million cars - will be almost evenly divided between large and small cars.

Sales statistics will show small cars outselling big ones next year, because General Motors introduced its new line of X-body subcompacts this spring but called them 1980 models.

The trend is detailed in a report written by the Commerce Department's chief economist, Courtenay M. Slater, and staff economist Joseph G. Carson.

They say small car sales would be passing big car output at an even faster rate if the auto industry could produce more little cars.

It costs millions to retool for smaller cars, they note; equipment made to machine V-8 engine blocks cannot handle four- or six-cylinder engines. Nor can suppliers quickly $&(WORD ILLEGIBLE switch their production to produce the parts needed for smaller cars.

General Motors gained the capacity to build another 150,000 small cars a year by opening a new plant in Oklahoma recently, but "a large further increase in the production of small cards cannot be achieved before the 1981 der of the year."

"Foreign producers are unable to increase production very much" either, the economists conclude, warning that the record sales pace of imported cars this spring "probably cannot be maintained during the remainder of the year.

Slater and Carson point out that big car sales bombed after the 1973-1974 Arab oil embargo. Although car sales in general rebounded, sales of full-size cars never returned to their 1973 levels.

The same thing is happening again, they say, "even if the shift (to small cars) is short-lived, it is unlikely that consumers will ever again purchase as many large cars as in 1978."

When the gas lines began to form this spring, consumers quickly turned from imported oil to imported cars, the government economists note.

The pace of import sales jumped from 2.1 million cars a year in January to 2.7 million a year by May and the imports' share of the market soared from under 18 percent to 24 percent in four months.

Sales of domestic compacts and subcompacts followed the same trend and by May, 60 percent of the autos sold in America - foreign and domestic - were small cars.

While small cars are sold on the basis of their gas mileage, tbe Commerce economists confirm auto executives' contentions that it is fuel supply that counts.

The proof is in Canada, they say, where gasoline prices have risen sharply since 1973, but supplies are plentiful. Sales of big cars in Canada are higher this year than last, and sales of imports are off sharply, in part because of depreciation of Canadian currency. CAPTION: Graph, Domestic Auto Production by Size Category, By Bill Perkins - The Washington Post