DETROIT — Sales of new cars and trucks plunged to their lowest level in more than a decade in June, as high gas prices and a weak economy kept American consumers away from dealer showrooms.

With the drop last month of more than 18 percent, automakers now expect to sell well below 15 million new vehicles this year, far fewer than the norm this decade of more than 16 million vehicles a year.

Detroit automakers were hit hard. Ford Motor was down 28 percent in June, General Motors was off 18 percent, and Chrysler dropped 36 percent.

Despite its sharp decline, G.M’s results were better than expected, which industry analysts attributed to a sales blitz with offers of zero-interest, long-term financing deals. The cut-rate loans helped G.M. retain its historic position as the top-selling United States automaker over Toyota, whose sales fell 21 percent.