DETROIT — U.S. auto sales in May brought the starkest signs yet that gas prices have shifted the market to smaller cars, as the top-selling Ford F-series truck was dethroned by cars from Toyota and Honda and as General Motors announced it was closing four truck and SUV plants.

GM said yesterday its sales fell 28 percent in May compared to a year earlier, with a 37 percent decline in truck and SUV sales and a 14 percent drop in car sales. At the automaker's annual meeting in Wilmington, Del., CEO Rick Wagoner said GM will close four truck and SUV plants in the U.S., Canada and Mexico by 2010, affecting 10,000 jobs.

"We at GM don't think this is a spike or a temporary shift," Wagoner said of the drop in truck sales.

Ford's sales fell 16 percent for the month, while Chrysler's sales were down 25 percent and Toyota's sales slipped 4 percent. Overall sales were down 11 percent compared to last May, according to Autodata Corp.

Honda, riding the wave of customers seeking better fuel efficiency, said its sales rose 18 percent; a 36 percent increase in car sales made up for an 8 percent decline in truck and SUV sales.

Nissan said its sales rose 8 percent, with a 19 percent increase in car sales offsetting a 10 percent decline in trucks.

High gas prices, the weak economy and low consumer confidence are taking their toll on larger vehicles. Ford Motor Co. sales chief Jim Farley said small and mid-size cars made up 47 percent of sales in May, up from 34 percent in February. The shift represents 1.5 million vehicles at Ford.

"May was a watershed month. We are, as an industry, catching up with the breathtaking choices customers are now making," Farley said in a conference call with media and analysts.

The Toyota Corolla and Camry and Honda Civic and Accord sedans each outsold the F-series truck, which saw monthly sales plummet 31 percent in May to 42,973. F-series trucks have been the best-selling trucks in the U.S. for 31 years and the best-selling vehicles overall for nearly as long.