The company says new models boosted car sales by 29 percent. But sales of the Chevrolet Silverado, GM's top-selling vehicle, fell almost 17 percent from a year ago.
Still, GM says last month was its best September since 2008.
The company expects total U.S. auto sales to hit an annual rate of 14.5 million for September. That would make it the second-best month of the year.
GM sold more than 210,000 vehicles in September. It was led by the Chevrolet Cruze compact car with a 43 percent increase. Chevrolet Sonic subcompact sales were five times September of last year.
All automakers report U.S. sales on Tuesday.