NEW YORK (CNN/Money) - U.S. auto sales fell for General Motors Corp. and Ford Motor Co. in September as the nation's top two automakers couldn't maintain their hot summer sales pace, but No. 3 Chrysler Group managed to post a gain.

GM reported sales of 309,600 vehicles in the month, down nearly 20 percent from 386,016 a year earlier. Two fewer sales days in the latest month contributed to the decline: GM said the drop in daily sales rate was only 13 percent.

"Our September sales performance was softer than expected," Bill Lovejoy, a GM group vice president, said. "However, when you consider our exceptional sales performance in July and August, lean inventories, and the competitive environment, we expected there would be some weakening in September sales."

GM's daily sales rate for light trucks, which include pickups, sport/utility vehicles and minivans, fell 8.4 percent, while car sales declined by 17.6 percent.

Chrysler, the North American unit of DaimlerChrysler AG, the world's No. 3 automaker, sold 165,397 new vehicles in September, up 9 percent from a year earlier despite the two fewer sales days in the latest month. The daily sales rate increased 18 percent year-to-year.

Car sales increased 18 percent on a daily rate basis and light trucks sales also grew 18 percent, despite an 11 percent decline for minivans.

Chrysler attributed the gain to competitive incentives as well as a warranty offer on new vehicles that was not matched by its major competitors. Chrysler has followed GM and Ford in offering popular zero-interest financing on many of its 2002 and 2003 vehicles.

At Ford, U.S. light vehicle sales fell 5 percent to 279,752 in the month, with car sales off 9.9 percent and light truck sales off 2.2 percent. The company counted the same number of sales dates in both months.

More than 80 percent of the decline came from lower sales of Ford's F-Series pickup trucks, among the nation's best-selling vehicles, whose sales were off 16.8 percent in the month.

The company also was hit by a 24.8 percent decline for Mercury, which the company attributed to lower sales to business customers through fleet sales. Industry-wide fleet sales have been off this year, particularly as car rental companies cut back on purchases following the drop in travel after Sept. 11.

The year-ago sales number included the hit sales took immediately after the Sept. 11 terrorist attacks, but also included about a week of the zero-interest financing that was introduced soon after the attack.

All the major auto companies offered zero-interest financing on some of their vehicles last month as well. Ford offered it on fewer 2003 models than did GM and Chrysler, although it offered it on more 2002 models still on dealer lots. Ford expanded its zero-interest offer to more vehicles Tuesday.

While Ford and GM lost ground in the month, some leading importers, including Toyota Motor Corp. (TM: down $0.95 to $50.15, Research, Estimates), Volkswagen and Hyundai Motor, posted improved September sales.

Shares of GM (GM: up $1.74 to $40.64, Research, Estimates) were higher despite the weak sales announcement, while Ford (F: up $0.09 to $9.89, Research, Estimates) reversed earlier declines to post gains. American depositary shares of DaimlerChrysler (DCX: up $2.84 to $36.35, Research, Estimates) also gained ground.