U.S. and foreign auto makers said yesterday their combined sales topped 10 million cars in the 1984 model year -- the best showing in five years and an increase of 17.2 percent.

Import sales hit a record number but the companies took just 23.2 percent of the U.S. market -- their smallest share in five years. In the 1983 model year, foreign firms grabbed 26.5 percent.

Combined sales by all auto makers in the United States were 10.31 million, compared with 8.79 million in 1983. It was the first time the firms have topped 10 million cars since 1979's performance of 10.8 million.

Import companies accounted for a record 2.39 million sales, up 2.9 percent from the previous model year. The drop in market share is the result of import restrictions and the growth in sales by U.S. firms.

The six American firms -- Ford Motor Co., General Motors Corp., Chrysler Corp., American Motors Corp., Volkswagen of America and American Honda -- reported sales of 7.91 million autos in the 12 months ended Sept. 30. This compares with 6.46 million in the 1983 model year. The 1984 performance was the best for the companies since 8.62 million cars were sold in 1979.