The nation's auto makers yesterday reported a 38.3 percent drop in new car sales for the Sept. 11-20 period, as sales failed to come close to last year's buying surge fueled by incentive programs.

The seven companies, General Motors Corp., Ford Motor Co., Chrysler Corp., Honda Motor Corp., Volkswagen of America, Nissan Motor Manufacturing U.S.A. and Toyota Motor Sales U.S.A., reported combined sales of 161,633 cars in the United States. This compares with 294,636 units sold in the year-earlier period. There were eight selling days this year, compared with nine in 1986.

For the period, GM's sales plunged 43.1 percent, while Ford posted a 31.2 percent drop from year ago levels. Chrysler, which acquired American Motors Corp. this year, showed a 51.4 percent drop from the total sales figure of last year. Sales of Honda's U.S.-built models rose 12.5 percent from a year ago. Volkswagen's domestic-made models fell 31 percent. Nissan's sales amounted to 3,313 units, compared with 1,995 a year ago. Toyota's sales were estimated at 2,873 units, with no year-ago comparison available.