a few months back, I started a thread about hypermileing and linked to a website found on msn about it, I have never done any modifications to my cars, but I have often thought of adding a boattail to my astro van or suburban to improve the drag coefficient that barn door out back is just literaly SUCKING my fuel mpg's down.
I have gotten my suburban down around 25MPG by changing my driving styles and putting larger diamater tires on the car. I use the truck for work, so hideous mods would not be allowed... but it's fun to think about it.
http://www.peachparts.com/shopforum/showthread.php?t=189642