"The U.S. is adding jobs, disposable income is rising, energy prices and interest rates remain low and business continues to invest, but the fact remains this has been a slow recovery," said Mustafa Mohatarem, GM's chief economist.
"The economy still has room to grow and so do auto sales, particularly now that the millennials are entering the workforce and starting households," he added.
Ford's total sales rose 23 percent to 221,599. Ford-brand SUV sales increased 27 percent, the best in 12 years, while trucks sales increased 23.2 percent.
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Fiat Chrysler's sales increased 14 percent to 193,019, boosted by the continued strength of its Jeep SUV brand. Jeep sales jumped 40 percent.
"On the back of a strong sales industry we were able to achieve our best September sales in 15 years and our 66th-consecutive month of year-over-year sales growth," said Reid Bigland, head of the company's U.S. sales.
While low gas prices seem likely to continue to help sales of SUVs and pickups such as the Chevrolet Silverado and Ford F-150, the days of near-zero interest rates look set to end as the Federal Reserve considers raising rates for the first time since 2006. Strong auto sales could help hasten that decision.
Edmunds.com has forecast a rise in sales for all major automakers except Germany's Volkswagen AG, which reports sales later in the day.
The world's biggest carmaker by sales faces up to $18 billion in penalties to go along with a severely dented reputation after admitted to U.S. regulators that it programmed its cars to cheat on emission tests.