Today Dr. Webber mentioned that if we were willing to sacrifice looks, we could gain 2-8% fuel economy. It reminded me of this article over at Autobloggreen. This guy took a '92 Honda Accord CX (which stock has a respectable ~48 mpg) and after about $400 in mods, is able to get maximum economy of ~95 mpg. That's a little more than 8%.
Of course the car looks really terrible.