A Census: Automobiles in the U.S. by Decade, 1900-1990

Taken from U.S. Historical Statistics:

Year # of automobiles
1900 8,000
1910 312,000
1920 9,239,200
1930 26,704,800
1940 32,453,200
1950 49,161,700
1960 73,857,800
1970 108,418,200
1980 155,796,200
1990 188,797,900