Struggling to comply with federal fuel-economy and emissions regulations even as Americans buy ever-bigger family vehicles, U.S. automakers have quietly begun experiments with oil companies to develop potentially cleaner fuels. Ford Motor Co. and General Motors Corp. have entered long-term partnerships with oil companies, and Chrysler Corp. plans to announce a program with immediate goals: testing vehicles over the next two years that will burn a promising type of synthetic fuel made from natural gas instead of crude oil.