Business Week: Since last December, when President Bush signed an energy bill that requires auto companies to achieve a 35-mpg fuel economy standard by 2020, with substantial improvements by 2015, auto executives have been gnashing their teeth while environmentalists have been flashing the V sign for victory. Three months later, it has become evident the road to greater fuel efficiency is full of potholes. For one thing, the new legislation adds a new level of complexity to the 35-year-old CAFE …