DETROIT—General Motors’ October U.S. sales plunged 45 percent, and Ford’s and Chrysler’s weren’t far behind, as low consumer confidence and tight credit combined to bring the industry’s sales to an “unsustainably weak level” that is the worst in 25 years.

Automakers sold 838,156 vehicles in October, 32 percent fewer than the same month last year and the worst performance since January 1991, according to Autodata Corp. The seasonally adjusted annual sales rate of 10.6 million vehicles was the lowest since February 1983.

“It’s really an unsustainably weak level for all manufacturers,” said GM’S Mike DiGiovanni. “This is .... something we really can’t sustain.”