Mid-Month U.S. Auto Sales Up 20.9% From a Year Ago : Economy: Much of the strength in the Sept. 11-20 period came from Japanese companies with U.S. factories.

DETROIT — Domestic auto makers reported a 20.9% increase in mid-September sales Thursday, allaying concerns of a reversal in the auto industry’s recovery.

The gains compared to the same period a year ago were shared almost equally between cars and trucks. Cars were up 20.5%, and trucks advanced 21.3%.

On an annualized basis, cars sold at a 6.9-million rate, more than 600,000 units above last year’s 6.3-million annual rate. Light trucks--pickups, minivans and sport utility vehicles--sold at an annualized rate of 4.5 million, about 500,000 units more than the same period last year.

Together, domestically built cars and trucks sold at an annualized rate of 11.4 million, slightly less than the 11.5-million annualized rate of the last 30 days.

Dean Witter Reynolds Inc. analyst Ronald Glantz said the mid-September period comparison is a good example of the industry’s recovery in sales to individual buyers, which are more profitable for auto makers than sales to large-scale buyers such as rental car companies.

“It was exactly now when you had the impact of the industry beginning to reduce sales to rental fleets,” he said. Last year was the first time in several years that the early production of new models did not go to fleets. The bulk of early shipments of 1994 models are now arriving on dealer lots.

Much of the strength in the Sept. 11-20 period came from Japanese companies with U.S. factories. Sales from the so-called transplants were up 58.1%, but the high percentage is deceiving because it is based on the relatively small share of U.S.-built Japanese cars and trucks.

Still, Japanese companies that have expanded their U.S.-built offerings posted especially strong numbers. Mitsubishi, which moved production of its Galant mid-size sedan from Japan to Illinois, and Nissan, which builds its Altima mid-size sedan in Tennessee, both showed 156% gains over year-ago sales. Nissan built the Altima’s predecessor, the Stanza, in Japan.