High gasoline prices and other economic woes have driven car-buyers in the U.S. to purchase smaller, more fuel-efficient vehicles lately. Last month, sales of compact and subcompact cars made up about 20 percent of total sales; in the mid 1990s, small cars accounted for only about one in eight cars sold in the country. Sales of vehicles with four-cylinder engines also outpaced sales of six-cylinder cars. “It’s easily the most dramatic segment shift I have witnessed in the market in my 31 years here,” said a Ford sales analyst. Meanwhile, sales of SUVs and trucks are seeing a sharp downward trend this year, with SUV sales plummeting 25 percent so far. Some market watchers are forecasting a continued shift away from big vehicles since gasoline prices are expected to remain high for a long while. “The era of the truck-based large SUVs is over,” said the CEO of the biggest auto retailer in the U.S.