These high-efficiency cars must get to Boston’s Greenfest, at City Hall Plaza, within three hours, using as little energy as possible. Some are electrics, some hybrids , and others use diesel or gasoline for fuel. But all cars will display how … We expect several contestants from the Progressive Auto -X Prize, which is offering multi-million dollar prizes in 2010 for efficient, marketable cars . Many of these cars go far beyond traditional concepts and some are even in …