The company blamed much of the drop from a year ago on reduced sales to rental car companies. But retail sales to individual buyers fell 3 per cent.
GM's sales continued to lag growth in the overall market. Analysts expected total U.S. sales to be up 11 per cent over the same month last year.
Sales of the Chevrolet Cruze compact fell more than 39 per cent. It was the top-selling compact in July of last year.
Last month GM warned that rental car sales would be down in July because sales came early this year.
GM began offering a money-back guarantee to buyers on July 10.