If you want:
http://www.cheatincorporated.net/Splashgoogle/gas.html?gclid=CLnM6bupg5ICFShMGgodgHiOUw
http://www.apdinc.com/GTAfuel.html
also, remove all things you don't need from your vehicle, such as the backseats, crap, trash, change-
remove as much weight as possible. Also, drive less. Walk more, bike more, take the bus more.