Early September auto sales slumped dramatically in comparison to the same period a year ago, with General Motors Corp. posting a 60.8 percent decline, the auto makers said yesterday.

Sales of all U.S.-made cars for Sept. 1-10 totaled 266,192, down 48.2 percent from 513,677 last year, the companies said.

Company spokesmen said the declines in the eight-day selling period were inevitable in comparison with surging sales caused by the introduction of bargain financing incentives last year. Sales in the eight-day selling period of September 1986 were the highest ever for GM and the second-highest ever for Ford Motor Co., company officials said.

GM sold 91,075 domestic-made autos in the period, down from 126,629 for Aug. 21-31 and 232,252 for Sept. 1-10, 1986.

Ford sold 51,243 U.S.-made autos, down from 66,515 for Aug. 21-31 and a 33 percent drop from 76,499 in the year ago period.

Chrysler sales totaled 22,659, including sales of recently acquired AMC models, down from 31,388 in the previous 10-day period and a 55.6 percent decline from combined Chrysler-AMC sales of 51,076 automobiles in the same period a year ago.