In the pre-dawn cold of a Michigan winter last January, the first 1980-model U.S. car rolled off the assembly line at General Motors Corp.'s Willow Run plant.

It was a dark-blue Chevrolet four-door compact, one of GM's new downsized front-wheel-drive cars that are expected to tighten the stranglehold of the No. 1 automaker on the U.S. market.

The new Chevy, one of four new models codenamed the X-cars, isn't due to be introduced until April 19. But GM's 1979 calendar year sales already were running 18 percent ahead of last year through the middle of February.

The automaker has captured more than 49 percent of the total U.S. car market, up from 45 percent over the same period a year ago.

With the average U.S. car now selling for about $7,000, one percent of the market is worth about $770 million. So GM's gain, sustained all year, will be worth an additional $3 billion in sales.

The increases come on top of GM's third consecutive year of record earnings in 1978, when the company made $3.3 billion.

GM's success has come at the expense of Ford Motor Co., whose penetration has shrunk from 24.2 percent to 22.6 percent, and troubled Chrysler Corp., which has slipped from an 11.1 percent share of the market to 10.2 percent.

Chryslaer lost $209.6 million last year and isn't sure if it will make a profit this year. Its problems led new company President Lee Iacocca recently to fire all three of the company's advertising agencies and hire Kenyon and Eckhardt Inc. away from his old employer, Ford.

American Mators Corp. has become so dispirited over the competition that it no longer releases 10 day sales figures, the industry's standard in measuring performance. The company now ranks fifth in U.S. production behind Volkswagen of America.

In an effort to upgrade the company's image, AMC President Paul Tippett has urged reporters to stop describing the company as "ailing and beleagured," and instead to use upbeat adjectives such as " aggressive and ambitious."

Despite tough talk by Ford that it is beginning to recapture lost ground, the company recently shut down two assembly plants in an effort to reduce inventories of compacts and intermediates.

Chrysler, which had a 114-day supply of cars on hand recently -- nearly twice the industry norm of 60 days -- is closing five of its eight U.S. assembly plants for one-week and two-week periods this month.

GM's success has drawn some backbitting from competitors. "Basically, they went out and stole fleet business from everybody else," says one rival company analyst. "Their retail sales aren't any better than anybody else."

But William O. Bourke, head of Ford's North American operations, conceded that Ford was outgunned by its bigger competitor. "Suddenly we work up and discovered they were doing to us what we'd done to them the prior two years," he said.

GM entered the 1979 model year with only one set of new cars -- the luxury front-wheel-drive Cadillac Eldorado, Oldsmobile Toronado and Buick Riviera, freshly restyled and downsized but still low-volume models. In a business where newness still sells, that could have been a sizable handicap.

Ford, by comparison, had the newly shrunken Ford LTD -- traditionally its bread-and-butter car -- as well as the redesigned Mustang, which was introduced with more than customary hullabaloo.

And Chrysler was getting ready to introduce the slimmed-down Chrysler New Yorker and Newport and Dodge St. Regis full-sized cars.

Knowing in advance the obstacles it had to overcome, GM determined to out-sell its competitors and developed some new tactics to do it. "It could have been rough," conceded Thomas Staudt, head of marketing for the Chevrolet division. "We thought we were facing a potentially very serious situation."

But Staudt added, "We were determined not to be found wanting in such a clear-cut test of our competence." Noted a competitor, "They got a little antsy because they didn't have anything else going for them except muscle so they turned the screws in their retail operation."

Chevy put on a huge push, which included: bringing 6,000 Chevrolet dealers to Detroit in August to detail marketing strategy, funding a $3 million ad campaign in September before Ford and Chrysler introduced their new cars, Allowing dealers to sell 1979 models before the official introduction date and running a dealer incentive program unusually early in the model year.

The effort paid off. Over the last three months of 1978, Chevy stretched its lead over the the Ford division from 319,000 cars to 509,000.

Oldsmobile, GM's second-largest-volume division, showed similar gains. For the last quarter of 1978, its sales ran 26 percent ahead of a year ago. Its January sales set a record for the month and were 29 percent higher than the same month in 1978.

Meanwhile, Ford was having a tough time getting started. The company let inventories of its 1978 models slip below desirable levels, leaving dealers with little stock to tide them over until the 1979s began coming in. As of Oct. 1, Ford and Lincoln Mercury dealers had 73,000 big cars in stock compared with 123,000 the year before.

Trouble-plagued Chrysler was having even more difficulties. The company, which has been pruning unprofitable foreign subsidiaries to generate cash, took two months to gear up production on its new full-sized line.

The new GM compacts -- the Chevrolet Citation, Pontiac Phoenix, Oldsmobile Omega and Buick Skylark -- are the source of considerable envy in the industry. They represent the third round of industry downsizing -- GM has led in all three -- and will cost $2.5 billion, which is a lot for GM and almost a prohibitive amount for anyone else.

The new cars are 20 inches shorter and 800 pounds lighter than their predecessors and will compete directly with the Fairmont and Mercury Zephyr and with Chrysler's slow-selling, recall-prone Plymouth Volare and Dodge Aspen.

Of course, GM has had trouble with new models before: Remember Chevrolet's Corvair and Vega? And GM's compacts in the past have not been huge successes.

But Bourke and everyone else in the industry are mindful of GM'S SUCCESS IN DOWNSIZING -- FROM THE Chevrolet to the Cadillac Seville. And if there was any question about the giant's marketing skill, that should have been answered this year.