The US government has long recongnized the need for more fuel-efficient vehicles. To that end, the government and domestic automakers - Chrysler, Ford and General Motors - formed a partnership four years ago to develop an affordable, mid-size vehicle that could get up to 80 miles a gallon while meeting a prevailing emission standards. After evaluating hundreds of technologies, the Partnership for a New Generation of Vehicles (PNGV) reached its first major milestone last year by selecting the technologies that show the most promise for meeting the program's goals and deadlines. But a major stumbling block may lie in the vehicle's affordability and its ability to meet emission standards, said a committee of the National Research Council in a new report.