Sadly, this form of analysis is a pipe dream, no pipeline pun intended. In the scenario that the Democratic Party of the US actually tackling the automobile industry for mileage requirements, it would first of all be many years before any significant portion of the vehicles on the road were of any consequential difference. Second, the production of all these new cars would require energy, and the alternative energies we hear about (Ethanol, for example) do not reduce but increase reliance on fossil fuels and greenhouse gas emissions-- you just don't have to see them as directly.