U.S. auto sales tumbled again in December, capping one of the worst years for the industry in decades and solidifying the view that more turmoil lies ahead in 2009.

For the month, sales of cars and light trucks fell 36% to 896,124 vehicles, according to Autodata Corp, a Woodcliff Lake, N.J., research firm. That is an improvement over both November and October; still, it was the fourth month in a row that sales failed to exceed one million vehicles.

For...