DETROIT -- U.S. auto sales fell sharply in September, enduring a tough hangover from this summer's Cash for Clunkers buying spree, according to The Associated Press.

General Motors Co. and Chrysler Group LLC posted the biggest slowdowns during the month, while Hyundai was the sole winner among big carmakers, reporting a 27 percent rise in sales from a year earlier.

"It was a more difficult month than we anticipated," Mark LaNeve, GM's vice president of U.S. sales, told reporters during a conference call."

The September slump for car and truck makers follows a heady summer. Automakers got a big lift in July and August from clunkers, which spurred sales of nearly 700,000 new vehicles. The government program's big discounts lured in many customers who otherwise would have waited until later in the year to walk into dealerships.

Now automakers are starting to feel the effect. GM's sales plunged 45 percent to 155,679 vehicles in September, compared with a year earlier. Chrysler sold only 62,197 vehicles last month, down 42 percent.