Volkswagen sales were off 12 percent, while Toyota sales were down 4 percent.
But Ford and Chrysler appeared to have outperformed the industry. Ford sales were up 6 percent, while sales at Chrysler rose 1 percent.
All automakers report U.S. sales on Tuesday. Chrysler and Ford officials joined many industry analysts in predicting that overall sales fell last month for the first time in more than two years. Analysts say a quirk in the calendar — not lower demand — is to blame.
Labor Day weekend is typically strong for U.S. autosales, and this year was no exception. But because the holiday came early, the auto industry counted all of that weekend's sales in its August tallies. That means September's sales will be missing the usual holiday boost.
Analysts say all the factors that have been driving this year's strong car sales — including low interest rates, the improving economy and the need to replace aging vehicles — remain in place. They see a strong close to the year.
"The market continues to move at a very healthy pace," said Erich Merkle, Ford's top U.S. sales analyst.
September U.S. sales are expected to total around 1.1 million, according to auto shopping site Edmunds.com. That would be a 4 percent decrease from last September and a 24 percent decrease from the blistering pace in August, which was the best month in more than six years.