Big tires and the stock gearing most manufacturers put in their rigs these days just don't play well together. So they put in multiple overdrive gears in the tranny...and the highest ratio never gets used. This is what happened to me. With the stock 3.73 gears my rig would only stay in overdrive on a level road with no headwind - or downhill. Any kind of grade or a good strong headwind and the tranny would shift down and the engine rpms would shoot up. I regeared to 4.56 and it made a world of difference. Now it stays in overdrive most of the time. Fuel mileage? It was 12 mpg with the 3.73 gears and now, after 50,000+ miles on the 4.56 gears, I'm still at 12 mpg. But the engine and transmission are much happier.
This of course is with a gasser engine. If you are talking a diesel with all its torque coming on at 1600 rpm, the 3.73 gears would have been fine.