Sales of U.S. made cars slumped 11 1/2 percent last month, compared with the same period last year, as buyers apparently moved to fuel-stingy imports out of worries over gasoline.

Import sales soared 24 percent, according to monthly figures reported today by manufacturers and importers.

Overall, American auto sales for the month were down 5.4 percent as the five major domestic manufacturers reported a sales decline to 763,702 from 862,940 a year earlier.

Imports were estimated at 222,000, compared with 178,900 for the same month last year. That meant the importers improved their share of the market from 17 percent in April 1978 to 22.4 percent this year, only a whisker under their record share of 22.5 percent last month.

U.S. built small cars-subcompacts and compacts-held their 53 percent share of the domestic market because of those same fuel worries. And though not part of the auto reports, sales of vans and recreational vehicles were "lousy" because of the same gasoline concerns, Chrysler President Lee A. Iacocca said.

Sales of all U.S.-owned compaines were down, but General Motors Corp. fell only 4.4 percent. Bouyed by the introduction of its new line of "X-body" compacts, GM's sales came in at 460,828, compared with the 481,912 it sold in the same month a year earlier.

Ford Motor Co. sold 192,273 cars, off 21 percent from 244,307; Chrysler Corp. sold 82,509, off 31 percent from 119,313; and American Motors Corp. sold 14,642, down 16 percent from 17,408. Volkswagen Manufacturing Co. sold 13,450 Pennsylvania-built Rabbits, a model not on sale last year.

That gave GM 60.3 percent of the April domestic market, Ford 25.2 percent, Chrysler 10.8 percent, AMC 1.9 percent and Volkswagen 1.8 percent.

April's results put the domestic manufacturers almost as even as it was possible to get with last year for the model year to date-2,948,324 cars vs. 2,918,658. Since there was one more selling day this year, the rate of sales per day was 0.02 percent ahead of last year-seven cars a day.

Analysts have commented there was no way the industry could match last year's land-office pace, and was bound to slow from a March that heated up with sales delayed by February blizzards.