Here is a good gear calculator.
http://www.ring-pinion.com/calculators.shtml
Plug in what you know and it will figure it out. DRIVE in a OD tranny is your 1:1 gear. On most V8's the sweet spot in that gear is about 2500RPM at 65mph. That's where most Gas engines usualy are running most effiecent. Also understand that what is printed on the side of that tire in reguards to size is not always what that tire really is.
The poor mans fix would be a set of towing tires (current) and a set of tall tires for the rest of the year as long as you are 2wd. Looks a little redneck but it would lower you engine RPM and probably far cheaper then changing the gears in the Diff. Then when you need the better pulling power you can change back and lower you effective gear ratio in your driveway in about 30 minutes. Problem is it will throw off your speedometer and on FI truck it may throw off the computer if you take this to far. 2 inches in tire hight is a conciderable amount of RPM for a given speed.
As for changing your gear. lowering the RPM does not always equate to better MPG. If the truck is lugging at 2200rpm and you have the throttle 3/4 open to maintain speed, But with the lower gear your turning 2800 RPM but only need 1/2 throttle to maintain that speed your probably using less fuel at 2800 rpm and the engine is actually having to work less because it has less resistance.