DETROIT — Sales of new cars and trucks in the United States plummeted in October to levels not seen in the auto industry in 25 years.

The stunning fall-off affected all automakers, as shaky consumer confidence and the inability of many eager shoppers to get loans because of tight credit drove sales down 31.9 percent during the month compared with the same period last year.

The grim results — particularly for General Motors, whose sales dropped by 45 percent during the month — raised new concerns about the chances of survival for Detroit’s troubled Big Three.

The auto figures add to the steady march of statistics that suggest the broader economy is grinding to a slower pace. A measure of overall manufacturing activity in the United States fell last month to its lowest level in 26 years, according to data released Monday. The Commerce Department also said that construction spending fell for the eighth time in 10 months in September.