DETROIT, JUNE 3 -- The nation's auto makers today reported a 17.7 percent drop in new car sales for May, as many of them half-heartedly promoted incentive programs at the start of the warm-weather buying season.

The eight companies -- General Motors Corp., 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 622,303 cars in the United States during the latest month.

The daily selling rate of 24,892 cars compares with 30,241 for the same period last year. There were 25 selling days in May this year, compared with 26 last year.

The annual rate for the industry during the month was 6.8 million cars, compared with 8.4 million last year. So far in 1987, the companies have sold more than 2.98 million cars, down 11.6 percent from the comparable 1986 period.

Import car sales in May were estimated at 260,000, down 5 percent from May 1986. Imports last month accounted for 29.5 percent of the overall car market; last year, they comprised 26.6 percent.

U.S. light truck sales last month were 328,170 units, 3.8 percent below year-ago levels. All U.S. auto makers except Honda, VW and Toyota build trucks domestically.

Industry giant GM posted a 31.3 percent decline in car sales, while Ford posted a 0.2 percent gain and Chrysler a 22 percent decline. AMC's sales fell 41.6 percent from year-ago levels.

GM's share among the eight auto makers slipped last month to 49 percent from 58.7 percent last year. Ford's share rose to 29 percent from 23.8 percent, while Chrysler's declined to 13 percent from 13.7 percent in May 1986.

Honda, which has been producing more model types domestically this year, posted a 67.8 percent increase, while Volkswagen's domestic-made models showed a 8.7 percent gain. Nissan's sales rose 1,558 percent from very low year-ago levels due to model changeover.

Toyota's sales were 4,463 units, with no year-ago comparison available because of its recent production startup. It began producing its FX subcompacts at its joint venture plant with GM in Fremont, Calif., in late 1986.

Thomas O'Grady, head of Integrated Automotive Resources Inc. in suburban Philadelphia, said the current cut-rate buyer incentive programs, especially those offered by GM and Chrysler, have been little advertised.

"They have a tag line on commercials, but people aren't aware of them as much," he said, noting that it could be because of overall cutbacks in advertising funds or auto makers reaching allocated budgets for certain models.

GM's cut-rate financing programs were extended on most models past the May 31 expiration date to June 12. A program on the Chevrolet Spectrum imported from Japan runs until the end of June. Ford's incentive offers run until June 15.

GM currently is evaluating a 10-week program under which it is offering about 2 million owners of GM vehicles in four marketing areas cash rebates of up to $1,500 in addition to other incentives if they buy a new GM car or light truck. It could be expanded on a national basis if successful.