It really comes down to load on the engine for a given driver. There are times where you can actully gain mpg by going higher.. and sometimes going lower. It honestly depends on too many things to give a one size fits all answer.
Anytime you increase mass of the wheels and/or tires you'll decrease milage. Finding the gear ratio to maximize that combo vs your driving style and terain is not easy. And because of the cost it can become subjective in short order!
I'd suggest gearing / tire-ing the truck based on how you want it to perform. Then live with the outcome. If your really wanting better milage stick with stock tires, roll up the windows, and drive 45mph. Or drive something else.