America's auto makers are rolling into 1979 as if their factories were running on "cruise control."

General Motors Corp. is predicting that the auto industry will top its record output of 1977 and 1978, and Ford Motor Co. is confident that 1979 will be at least the third-best car sales year ever.

But 1979 is a labor negotiations year for the auto industry, and a strike this fall could sideswipe those predictions.

The industry's output also might slow slightly as President Carter puts the brakes on inflation, Ford executives say, but they see at most a 4 percent decline in sales. Chrysler Corp. and American Motors Corp. don't argue with those assessments and probably have the most to fear if the economy slows down and Americans curb their car buying.

Both GM's forecast that Americans will buy 15.4 million new cars and trucks and Ford's prediction of a 14.8-million-unit year assume that imported cars will take a smaller market share for the third year in a row.

The continuing decline of the dollar will make the competitive Japanese and German cars more expensive, and the American auto makers expect to recapture sales with their new fuel-efficient cars.

As the cost of imported cars has climbed, Detroit has increased its own sticker prices, boosting the cost of cars that compete with imports in lock step with the increasing cost of the competition. Since last summer, the Big Four have boosted prices of 1979 models by 6.5 percent, or more than $450 a car. That's a full percentage point more than last year's increases, but it still complies with President Carter's inflation-fighting guidelines, government spokesmen say.

One reason Detroit is eager to raise prices is to recapture the record capital expenditures required by the industry's "downsizing" program of creating smaller cars in response to government fuel-economy standards and the car-buying public's rejection of gas guzzlers.

Downsizing will produce several major new models this year:

Chrysler's sporty two-door-coupe version of the Omni and Horizon is being introduced to give dealers a full line of cars to compete with the popular Toyota Celica and Volkswagen Scirocco. Omni and Horizon introductions were successful despite Consumer Reports magazine's criticism of the car's emergency handling.

Ford's new Capri and Mustang, smaller, faster, more economical variations on the powertrain introduced last year on the Ford Fairmont and Mercury Zephyr, are now rolling off the assembly line. Ford sold more than 400,000 Fairmonts the first year of production, the best first-year sales ever for a new model.

General Motor's new line of front- wheel drive conpacts that will be introduced in the spring, replacing the Chevrolet Nova and its look-alikes at other GM divisions. GM's downsized midsize cars introduced last year -- Chevrolet Monte Carlo, etc. -- were not as successful as Ford's Fairmont, in part because buyers didn't go for the hatchback styling of some models.

The new models should increase competition dramatically among the American manufacturers. Ford is looking for the new Mustang to repeat the success of the original pony car, and GM has even higher hopes and two new plants staked on its front-wheel-drive family.

Chrysler's corporate goal, even before it hired the Mustang's creator, Lee Iacocca, as president, was to increase its market share by 1 1/2 or 2 percentage points, returning to its traditional 15 percent position.

American Motors probably would be content if its total sales were as much as the sales gain Chrysler is going for.AMC captured less than 2 percent of the market last year, and this year expects to sell between 185,000 and 200,000 cars, and for the first time ever could sell more Jeeps than passenger cars.

Were it not for Jeep, AMC would be in danger of losing its role as the nation's No. 4 manufacturer to Volks-wagen. Despite Detroit's confidence that the imports' share of the market will decrease, VW is projecting a 24 percent sales increase and expects to sell 300,000 cars in 1979. It builds Rabbit subcompacts in New Stanton, Pa.

Mini pickups are also on the drawing boards to replace the Ford Courier (made by Mazda) and the Chevrolet Luv (produced by GM's Japanese affiliate, Izusu). The captive imports are selling well, but after 1979 imports can't be counted by American manufactuers in the federally mandated corporate average fuel economy (CAFE) standards. That's why the Mercury Capri, once imported from Germany, now is being built here.

Complying with CAFE or getting the government to change the standards will be the biggest challenge this year for the auto makers. This year's cars have to average 19 miles a gallon -- a goal Ford could have problems with if its small car sales are soft -- but the target moves to 27.5 mpg in 1985.

Only Volkswagen aims at that target with confidence, and the other American car makers are seeking to scale down the mileage figure or stretch out the time frame for meeting it. Urging the National Highway Traffic Safety Administration to set a lower goal, Ford warned that otherwise "consumers will find the interior space, utility and other dimensions of most cars during the 1980s will be below the level they are accustomed to."

Ford is leading that fight, and GM has the most at stake in another major regulatory fight that is brewing over diesel pollution. The Environmental Protection Agency is about to propose new diesel exhaust emission standards that could throw a monkey wrench into GM's plans to use diesel engines in its big cars as a way of meeting the government's mileage standards.