The Mini Cooper had a .35 drag coefficient. That is why its fuel economy isn't very good for its size and engine displacement. The new ones for 2007 will have a .33 Cd, which contributes to their increased fuel economy.
Due to the ability to be tapered back longer, it is easier to make an aerodynamically efficient big car than it is to make an aerodynamically efficient small car. It may seem counter intuitive, but IMO, the route to a high FE design that Americans will buy is to bring back the big car, make it aerodynamic as all hell, and put a big engine in it.