The switch to smaller, more fuel-efficient vehicles has been building in recent years, but has accelerated recently with the advent of $3.50-a-gallon gas. At the same time, sales of pickup trucks and large sport utility vehicles have dropped sharply.

In another first, fuel-sipping four-cylinder engines surpassed six-cylinder models in popularity in April.

“It’s easily the most dramatic segment shift I have witnessed in the market in my 31 years here,” said George Pipas, chief sales analyst for the Ford Motor Company.

The trend toward smaller and lighter vehicles with better mileage is a blow to Detroit automakers, which offer fewer such models than Asian carmakers like Toyota and Honda. Moreover, the decline of S.U.V.’s and pickups has curtailed the biggest source of profits for General Motors, Ford and Chrysler.

Once considered an unattractive and cheap alternative to large cars and S.U.V.’s, compacts have become the new star of the showroom at a time when overall industry sales are falling.

Sales of Toyota’s subcompact Yaris increased 46 percent, and Honda’s tiny Fit had a record month. Ford’s compact Focus model jumped 32 percent in April from a year earlier. All those models are rated at more than 30 miles per gallon for highway driving...

... “The era of the truck-based large S.U.V.’s is over,” said Michael Jackson, chief executive of AutoNation, the nation’s largest auto retailer.