To see how tire size affects engine rpm, go to www.4lo.com
and check out the gearing calculators. You need to keep the engine up to 2500-3000 rpm at cruising speed (say 70-75 mph, if that's how you drive). A lot more (or less) and mileage will suck. If you have a lot bigger tires than stock, it's the same as reducing the gear ratio. That in turn over works the engine. If you have stock gears, you need to keep very close to stock tire size. If you put big tires on, you have to change the gears (in front AND rear -- they MUST be the same in a 4x4) to compensate for the tire size. Like I said, use the gear and speed calculators on www.4lo.com
to see how tire size really changes things. You can't change one thing without affecting others.