DETROIT — Americans bought more cars in May than in any other month this year, drawn by fire sale prices that pushed General Motors and Chrysler's sales above expectations despite their forays into bankruptcy. Overall sales were still 34 percent lower than a year ago.

But low gas prices encouraged the sale of bigger vehicles while small cars stacked up on dealer lots. That could be a problem for the Obama administration, if the demand for more fuel-efficient vehicles drops just as it is forcing the U.S. auto industry to produce more of them.

"The great migration away from fuel efficiency is once again under way," said Mike Jackson, chairman and chief executive of AutoNation, the nation's largest automotive retailer. "The price of gasoline determines the type of vehicles consumers buy. Period."

All major automakers including Toyota and Honda posted drops from last year, with Ford benefiting from the financial woes of its Detroit competitors and recording the smallest decline at 24 percent.

Almost every small-car model saw significant drops in sales compared with May of last year, which was a record month for many models as gas headed for $4 per gallon.

Honda's Civic was off 61 percent, Ford saw Focus sales drop 54 percent, and GM's Chevrolet Cobalt was down 52 percent. Even Toyota's Corolla, the perennial small-car leader, saw sales drop 55 percent.

Yet sales of Ford's midsize Fusion jumped 9 percent, with four-cylinder engines making up 70 percent of the sales. Chevrolet's Malibu sales were down 11 percent, a far lower decline than the rest of the market.