Think of it like this.
Your motor has a certain powerband. It was designed for a certain transmission ratio, gear ratio, and tire ratio. When your putting power to bigger tires with your stock set-up, you have to rev your motor more to get the torque needed to move you along. So in effect you will get poor mileage.
Now on the other hand lowering your gear ratio (Increasing the number) will compensate for this, but I am not sure If there is a perfect gear and pinion that would make your truck perform like stock after a set of larger tires. So you will not get stock gas mileage with that.
For example. I have 43ring 10pinion. I have 4:30 gears. I have 31" tires. If I go to a 32 inch tire I will throw off my powerband and I will have to regear to compensate. The next closest gear is 4:56.
4:56 will give me more power and more revs, but it will not bring it to a stock configuration.
If I were to go with 33" tires 4:56 would not be enough to bring it to stock and I would have to go over with a 4:88.
So the moral of this story is that if you get bigger tires no matter what you do fuel consumption will be different, even if it is ever so slight. The question you must ask yourself is. Do I only want to do this job half-way and not re-gear? Or do I want to add bigger tires and make my truck a little peppier in the process.
If your a die hard 4 wheeler you will go with re-gearing with bigger tires. To me the benefits. More power, more clearance is the only way to do it.