Found this article on wired.com about the "Aerocivic." According to the article, the guy who built it spent about $400 on materials, but now gets up to 95 MPG.
Some links from the article:
A thread the owner started on ecomodder forum.
Pictures of the car and the building process.
How he measured his coefficient of drag.
A German site showing pre-WWII German aerodynamic experiments (hit the tour button at the bottom to see lots of pictures).
I realize his testing methods aren't the most scientific, and that his photo gallery states 80 MPG and his user on the forum states 64 MPG. Still some interesting reading, and gives a good visualization of aerodynamic mods.
Boy is it ugly, though.