Festiva FTW!
I've put less than a hundred bucks worth of mods on my lowly Festiva, and I average about 60 mpg at this point. Seventy of it was just for the Kammback, because I was too lazy to build my own.
Bought the Black Widow for a thousand bucks over two years ago, new tires were 33 bucks apiece, a new radiator was a hundred bucks, etc. . . . parts are cheap for this old beater, and it hasn't given me any trouble in two years of daily driving!
This brick shaped little toy car is prime for ecomodding, with one of the most fuel efficient 4-cly gas engines ever made hiding beneath the boxy hood. You can usually find yourself a 5-speed EFI for less than 1500 bucks, and then you can coast your way to 75% over EPA averages all day long, my friend!
FYI, they're everywhere, and cheap!
http://honolulu.craigslist.org/search/cta?query=festiva
http://maine.craigslist.org/cto/4561388527.html
http://wichitafalls.craigslist.org/cto/4608340308.html