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