U.S. auto makers reported today a 16.8 percent drop in car sales for mid-October as some top firms tried to stimulate the market by again offering cut-rate financing on some models.

The seven companies, General Motors Corp., Ford Motor Co., Chrysler Corp., Honda Motor Corp., American Motors Corp., Volkswagen of America and Nissan Motor Manufacturing USA, reported combined sales of 162,726 cars in the United States during the latest period, Oct. 11-20.

This compares with 220,112 units sold in the year-ago period.

A low-cost financing campaign earlier this year successfully boosted sales, although not without costs. GM, for instance, earlier this week reported its first operating loss since the third quarter of 1982. GM's operating loss for the third quarter this year was $20.9 million.

The daily selling rate for the most recent period was 20,341 cars, compared with 24,457 for the same period last year.

There were eight selling days this year, compared with nine last year.

The annual rate for the industry during the period was 5.4 million cars, compared with a soft 6.6 million last year.

Effects of a GM of Canada strike a year ago, which lasted from Oct. 17 through Oct. 29, affected GM's year-ago figures, company spokesman Harold Jackson said.

Compared with last year, GM's market share remained about even at an average 54.7 percent of the total market. Ford's share, however, dropped to 22.5 percent from an abnormally high 27.8 percent a year ago, possibly because of the GM strike.

Chrysler's share this period was a high 16.1 percent, compared with 13.3 percent a year ago.

So far in 1985, the firms have sold 6.89 million cars, up 5.8 percent from the almost 6.54 million units in the 1984 period.

During the period, GM, Ford and AMC had, in effect, 8.8 percent financing on certain 1985 models headed for a major restylings in 1986, and selected 1985 and 1986 models equipped with standard transmissions.

GM's and Ford's programs, which expire near the end of November, are aimed at selling more fuel efficient models to help them meet goverment-mandated fuel mileage requirements.