Also really depends on your driving habits. Ive got a 91.5 w250 nv4500 5 speed sitting on 37" military tires on steel wheels with 3.55 gears, i'm averaging 23.1 on my fuelly app. Thats on a 60 mile round trip to work everyday with atleast 10 stop signs and lights, and running between 45 and 55 mph. But 99% of the time i drive like the oldest grandpa you can think of. Oh and the truck has just a shaved pin, fuel screw in a few turns, and a h1c with a 12cm housing. Now i don't have any mpg numbers from all stock but i sure cant complain about 23 mpg on 37's. There isn't any scientific numbers to back it up but if you ask do i think the 37's help my mileage, i'll say yes. One of these days im gonna find a set of 33's or 35's to see what the difference is.
There are so many factors involved, just like was said, rolling resistance is one. If you decide to try it make sure to use a good street tire on an aluminum wheel for the least amount of weight and rolling resistance. No super swamper boggers on steel wheels.