General Motors Corp's five car divisions will officially unveil their 1978 models at national news previews this week.

Four of the five GM car divisions will show genuinely new, sharply downsized intermediates that are already widely acknowledged as the second generation of gasoline-minded U.S.-made vehicles since the Arab oil embargo.

F. James McDonald, executive vice president for North American automotive operations, briefed newsmen here Friday on the family of new cars at a low key press conference.

He said they will weigh 550 to 825 pounds less, be 9 to 18 inches shorter overall and yield 2.5 to 2.8 miles per gallon more than the comparable 1977 model.

McDonald also hoisted some surprises for the 1973 "A" and "A special" bodies which are essentially the Chevrolet Malibu, Old-mobile Cutlass, Buick Regal and Pontiac LeMans.

Those are:

The rear windows on the four-door models won't roll down. They'll be sealed shut and ventilation will come from vent windows just to the rear of them.

V-6 engines are coming on strong and will be the standard power plant in all the GM intermediates with V-3s optional equipment.

The spare tires will be for temporary use only, and so marked with large white letters on the side. They'll be inflated to 60 pounds of pressure and weigh 14 pounds less than the normal tires.

There'll be more difference among the divisions' intermediates. Oldsmobile and Buick '73 intermediates will share some features distinctly different from those of the Pontiac and Chevrolet intermediates.

McDonald said GM will be doing a major corporate advertising effort on the intermediates, a relatively rare activity but one that was done for the larger "B" and "C" body full-size cars last year.

"This is the first year we have to meet the fuel economy standards. We had a year of practice last year and an 18.1 mpg forecast actually came out at 17.8 mpg" for all GM cars sold, he said.

"We think we'll be between 18.6 and 18.9 mpg" for 1978, he said.

He defended the fixed rear windows vigorously, arguing that most rear door windows now don't roll down completely, the new method substantially increases rear seat room, and occupant vision also is improved.

At the same time the '78 "A" cars are five to seven inches narrower on the outside, he said.

"As you move toward the 27.5 miles per gallon required in 1985, there are just some things you have to do to try to keep passenger comfort and luggage room and meet fuel economy and emissions limits," he said.

"You might as well get started now," he said. Making the temporary, high-pressure spare tire standard equipment is another of those decisions, he said.