Ford Motor Company backpedaled yesterday on its promise to increase the fuel economy of sport utility vehicles 25 percent by 2020. It now says it will continue to try to improve gas mileage but will not set a fixed deadline for reaching the 25-percent goal. The company chalked up the change in plan to technological difficulties and the lack of federal tax credits for improved efficiency. The decision is likely to have ramifications throughout the industry, as the initial Ford pledge spurred General Motors and the Chrysler Group to follow suit. Environmentalists reacted with confusion and dismay to the news: “What Ford is doing is telling the American people that we can’t trust Ford’s commitments; they keep moving the goalposts back when the deadline nears,” said the Sierra Club’s Daniel Becker.