Efficiency is a large stumbling block if you're looking for a way to replace gasoline. It's pretty hard to better such an exceptional fuel, and several alternatives show promise but are nagged by inefficiencies or cost, and usually both.

In yet another move demonstrating that Porsche is capitulating to the greener crowd, an industry newsletter is reporting that the German automaker's executives have overcome their long-time reservations about diesel powerplants and are considering such a vehicle for their lineup by 2010.