though it's true you can make it more efficient by tweeking the front (at risk of overheating), the real gains are at the back of the vehicle. Years ago NASA did some research on a rolling van and was able to show that the front of the vehicle is 'relatively' unimportant compared to the back. IF you are looking to REALLY make it more efficient, what you need is a big ole bob-tail on the back! Air will naturally pile up on the front of the vehicle, and kind of make it's own aerodynamic shape. However, at the rea of the vehicle, there is a huge amount of drag (speaking about it as a blunt body). The flat back is just about the worst shape possible. Putting a tail on it will make that transition smoother and reduce drag significantly. Do a little search on modding a car for high mpg and you'll see some really good ideas
wheel covers (flat disks)
wheel well covers (including the front, which much follow the wheel)
fairing for the wheels and
belly pans are all high on the list of things that have the biggest influence.
The big problems is:
1) getting the manufacturers to follow form fits function
2) getting the public to accept it
The physics isn't that hard, but overcoming peoples idea of beauty and aesthetics is! IF you put an aerodynamic body on the smart it would return 55-60 mpg on the highway. In fact if you look up Mercedes Benz bionic car you will see they have already done the research on this to come up with a vehicle that has a good interior volume, and still maintains an outstandingly low coefficient of drag. A car shaped like this with a good diesel engine (say turbo 800 cc? sound familiar?) would return well in excess of 70 mpg.
Image Search Results for mercedes benz bionic car
File:Mercedes-Benz bionic car.jpg - Wikipedia, the free encyclopedia
The Bionic Car project
for your reading pleasure you can see what others have done (I don't think anyone has pushed the fortwo to the limits talked about here -- you could be the first!)
65+ vehicle modifications for better MPG -- MetroMPG.com