Just for reference, I have 31's and 3.73's....and do alot of driving. About 80% is highway, and the other part is city. I usually go through a tank and a half a week or slightly less. Power is fairly good on the highway too, in town it sucks.
My low average is 16.12mpg, and my high average is 16.51mpg. This is 65-70mph on the highway, and mixed speeds/accelerations in town. Tank average is 320-330. Best every with my current setup was few months back, 16.7mpg, 334 miles.
To me this is pretty astounding considering my truck is as aerodynamic as a refrigerator (don't believe me? go look at my build thread).
I bet my numbers would be slightly lower if I did mostly city driving....and when I go to 33's I probably won't be getting in the sixteens anymore...