Pontiac Motor introduced its 1978 model new cars here today, with most of the attention focused on its redesigned, sharply lighter, small-on-the-outside intermediate LeMans, Grand LeMans and Grand Prix.

The new models are 530 to 925 pounds lighter - weighing less than the comparable compact Ventura models - 8 to 17 inches shorter, and substantially narrower.

Like the other new General Motors Corp. "A" body cars, they have rear windows that will not roll down and have hegligible to interesting increases in leg room, hip room, and head room.

Alex C. Mair, general manager of Pontiac, made these points at a press conference:

Despite a 600-700 pound average weight reduction in the intermediates - "a good half-inch steel" - he expects to consume an equal amount of the metal in the coming model year because of increased sales.

There's no diesel Pontiac in the immediate future, but "by 1985 or so we will need more diesels."

P There is definitely a subcompact Pontiac, less than about 1,800 pounds, coming, but it may be five years off.

When the temporary spare tire, standard on all GM 1978 model intermediates, is used, the high-pressure and somewhat bike-like tire tilts that corner of the car down about half an inch.

Mair was asked what percentage of new cars will have front engine, front-wheel drive in 1985. "I'm going to guess that it'll be a third," he said.

Such guesses by people like Mair, an engineer, are taken seriously in Detroit.

Asked when Pontiac will have its first front-wheel drive car, he said "Oh, it'll be along . . . sometimes." It is an open secret in the auto industry that when GM downsizes its compacts, like the Pontiac Venture and Chevrolet Nova, in about 18 months they will get a front-wheel drive system.

Mair was asked about the continuing auto industry optimism about the economy and new car sales for next year. The question was raised in the light of the gloom rising from Wall Street.

"We are an optimistic industry, we do have to plan far ahead," Mair said. Significantly, he refused to back off from the GM's rosy predictions.

The average fuel economy for all 1978 model Pontiac will be about 19 miles per gallon, he said. Last fall Pontiac introduced two new engines specifically designed with fuel economy in mind.