General Motors Corp. Chairman Roger B. Smith predicted yesterday that 15.4 million cars and trucks will be sold in the United States by the end of the calendar year, topping the old record of 15.3 million vehicles sold in 1978.

Auto makers doing business in the U.S. market also have "a real shot of surpassing 15 million units once again" in 1986, and maybe setting "another new record if everything works out the way we hope it will," Smith said at a Detroit news conference yesterday.

Smith's predictions may come true this year, largely because of a huge consumer response to incentive programs -- 7.5 and 7.7 percent financing rates -- put in place by domestic manufacturers late last month.

The incentive programs yielded a record 405,080 sales of domestic-made cars for the 10-day period ending Aug. 31, according to Automotive News, a Detroit-based auto trade journal.

If Smith's forecast is accurate for 1986, it'll be because of an increase in import sales and of Japanese-nameplate cars and trucks made in this country, according to some auto industry analysts.

"Hardly a month seems to go by when another Japanese company is not announcing a new U.S. assembly facility," Merrill Lynch, Pierce, Fenner & Smith Inc. said in its latest auto industry report.

The Japanese now have potential capacity to produce 1.5 million cars and trucks annually in North America, with most of that production aimed at the U.S. market, the Merrill Lynch report said.

Japanese imports and Japanese-nameplate cars assembled in the United States accounted for 20.3 percent of the cars sold in this country in the first seven months of 1985, up from an 18.6 percent share for the same period a year ago, Merrill Lynch said.

The Japanese conceivably could increase their share, and probably will. But overall U.S. car and truck sales probably will drop 3 percent in 1986, to 14.8 million units, "because economic growth" in the United States "will be too slow" to sustain consumer confidence, Merrill Lynch said.

Smith said, however, that he believes that the 1986 product lineup will keep pulling buyers into showrooms, with or without incentives.

"The products are there," the GM chairman told reporters before addressing the Detroit Economic Club. GM, he said, "will have an outstanding model lineup with a number of all-new cars."

Smith said he believes that GM could again account for about 59 percent of the U.S. market. The company dropped to about 57 percent of the market in recent months.

On other matters, Smith predicted that his company will lead the auto industry into the 21st century in using computers to improve factories, offices and products.

GM's new Saturn Corp., which is expected to start producing up to 500,000 subcompact cars a year by 1990, already is phasing in a paperless expense report system, Smith said. Other Saturn computer functions coming on line "could soon make engineers 12 times more productive on some assignments and all but eliminate chances for error," Smith said.