My favorite "body bolted on existing frame of semi-modern car" is the Smyth Performance G3F, which is discussed in this thread:
http://ecomodder.com/forum/showthrea...tdi-14154.html. It may not be "super-aerodynamic" but it is (hopefully) more aerodynamic than the Jetta upon which it is based. Considering I can get 60mpg tanks with a Jetta body, I'd guess that 70 or even 80mpg tanks would be possible (maybe even more) with the G3F body.