Well first would have to be exhaust( bigger or straight pipe which ever you prefer), keep your foot out of it and try not to go higher than 2000 rpms, get a chip or a tuner, and an intake mod wouldn't hurt, with all of that I would just about say you would see at least a 5 mpg increase if done properly. Most people that do that to their truck usually don't like to be slow, they like to race and accelerate just to feel the power these monsters have. Just my .02 cents