The downsizing revolution in American cars will pass a key point shortly after Jan. 1, 1979, when Ford Motor Co. puts an all-plastic hood on a standard production model.

It will be the first car with "a plastic hood between metal fenders," according to Robert Gerrity, general manager of Ford's plastics, paint and vinyl division. "Between metal fenders," is a reference to the Chevrolet Corvette, a limited-volume car with a fiberglass plastic body. There have been other plastic hoods, but before now produced with a metal-supporting underbody.

Gerrity said Ford will have the capacity to produce 60,000 of the hoods annually and indicated they will probably be used on company's heavier cars.

In an equally striking statement, he said Ford will have an plastic automobile frame member - the part of an automobile whose role is that of the human skeleton - "well before 1985" and would "like to have it in a couple of years."

Automobile frames are the heaviest, strongest steel parts in a car.

The domestic auto makers have been downsizing their cars at an accelerated rate, starting just before the Arab oil embargo. Until now, those changes have largely been standard shrinkage of materials and re-designs.

Gerrity made most of his remarks recently to the Detroit Auto Writer Group (DAWG), which is largely composed of magazine reporters in Detroit.

He said that in 1965 "roughly 30 pounds of plastics" were on Ford cars. "Last year the average North America model had 167 pounds, and a 1979 model will have on the average 190 pounds.

"We're forecasting 280 to 350 pounds per car by 1985," Gerrity said.

Frank Daley, director of manufacturing development for General Motor Corp., told the auto writers the typical 1979 GM car contains 185 pounds of plastics, 108 pound of aluminum, 2,083 pounds of steel and 573 pounds of cast iron.

To meet the federal law requiring 27.5 miles per gallon for every auto manufacturer's fleet in 1985, the composite GM car will "be using somewhere between 240 and 300 pounds or plastic, 200 to 240 pounds of aluminum some 15 pounds of steel and around 35 pounds of cast iron," Daley said.