I put a gibson side sweep on my 3.0 manual and gained an average of 1.5mpg. Keeping a clean air filter keeps my MPG around 20-21. Before exhaust and consistant air filter changes I would get 18mpg on average. When I did all of the calculations for MPG gain my driving was consistant for the most part. To and from school all country roads.
I shift at 2500 RPM, run Mobil 1 synthetic oil, have 265/75/16's, 2wd, have a 200lb's worth of audio behind me, drive with the window halfway down most of the time, and use Fram filters. Even if I get on it ALL the time it never goes below 17mpg.
I am hoping to be able to go over 20mpg after this summer. I am either going to finish my intake or just remove the scilencer, replace spark plugs and wires (60,000 miles, time to change the plugs), and change the diff fluid to synthetic (LSD has stopped engaging when on moderate-extreme angles).