I never could understand the whole "Big tire" thing,,makes no sense especially if its a road warrior! Larger tires do not help you unless you are driving in alot of loose dirt/rock/sand crawling, we know there are alot of rocks to crawl over on the way to work! If your in Mud, a skinny tire is best, the only way a large wide tire helps in mud is if you have alot of HP and rpm's to skip over the deep mud.
Since I drive mostly on the streets, I have a stock lift, I don't race off road anymore and I work my truck so I have commercial tires (33") same size as stock.
I don't get trucks with gas engines anymore because of the efficiency of them, not just fuel, but power and torque which matters to me. I do have a few very high HP cars but then I race them...
To get mileage from an engine there is alot to think about, gearing, tire mass, vehicle weight and aerodynamics. Common sense would tell you that bigger tires, lower aerodynamics factor would affect your MPG's. You want to get your mileage back? get skinnier tires and drop that truck back down!
You would also need to make your engine as fuel efficient as possible, but that is another story.