DETROIT, AUG. 25 -- The nation's auto makers today reported a 17.8 percent gain in new car sales for the Aug. 11-20 period as top manufacturers followed industry leader General Motors Corp.'s offer of record-low financing rates to clear out a backlog of unsold vehicles.

The eight companies -- GM, Ford Motor Co., Chrysler Corp., Honda Motor Corp., American Motors Corp., Volkswagen of America, Nissan Motor Manufacturing U.S.A. and Toyota Motor Sales U.S.A. -- reported combined sales of 235,866 cars in the United States during the latest period.

This compares with 200,282 units sold in the year-ago period when Toyota was not producing cars domestically, and car makers were offering incentive rates as low as 5.9 percent.

The daily selling rate of 26,207 cars for the latest period compares with 22,254 for the same period last year. There were nine selling days both this year and last.

Light truck sales among the manufacturers that produce them domestically totaled 110,351 units, up 13.2 percent from a year ago.

The annual rate for the car industry during the period was a strong 9.2 million cars, compared with 7.8 million last year. So far in 1987, the firms have sold about 4.65 million cars, off about 9.5 percent from 5.14 million units sold in the comparable 1986 period.

For the period, GM's sales rose 18.4 percent from year-ago levels. The top auto maker said its latest incentive program, which since Aug. 6 has been offering annual finance rates as low as 1.9 percent on 24-month loans, helped it reach the highest car and truck sales level for any mid-month period so far this year.

However, industry analysts said part of GM's gain was because sales were poor in the year-ago period just prior to the 2.9 percent offer. Last year, GM's massive 2.9 percent financing program, which other car makers quickly matched, did not begin until Aug. 28.

"They are still coming off a bad period," said Thomas O'Grady, head of Integrated Automotive Resources Inc. in suburban Philadelphia.

Ford, which quickly matched GM's offer, posted a 24 percent gain in car sales. "Ford just keeps on rolling on," O'Grady said. "It can only mean that profits and sales will be an all-time high this year for them."

Chrysler's sales declined 18.3 percent from year-ago levels despite the incentives. AMC, now a wholly owned subsidiary of Chrysler called Jeep-Eagle, reported a 59.6 percent drop in car sales.

Sales of Honda's U.S.-built models rose 81.9 percent, partly because it is now building more model types domestically.

Volkswagen's domestic-made models showed a 28.9 percent decline in sales.

Nissan sold 5,527 cars in the latest period. A year ago, it had no sales of its U.S.-made cars because it was changing over to a new model.

Toyota's sales were estimated at 1,188 units, with no year-ago comparison available because of its recent production startup. Toyota began producing its FX subcompacts at its joint venture plant with GM in Fremont, Calif., in late 1986.