By refocusing on small cars and de-emphasizing the gas-guzzlers that had long sustained the industry, General Motors and Ford in particular are preserving jobs and positioning themselves to prosper. Their efforts are already paying off in the marketplace. Ford's tiny Fiesta is the best-selling subcompact in the United States this year, and G.M.'s Chevrolet Cruze outsold every other compact car in America last month except the segment-leading Honda Civic. Nearly one in four vehicles sold in the United States in April was a compact or subcompact car, compared with one in eight a decade ago. Of the small cars sold in April, about 27 percent were American models, compared with 20 percent a year earlier.