Don't forget the size tires you run will effect your speed/rpm figures. depending on the tire size you now have,taller tires could lower your rpm's considerably at the same speed. i.e.going from a 235/70-15 to a 235/85-16could give about the same effect to your rpm's as changing from a 4.11 rear end ratio to a 3.73 ratio,and might cost quite a lot less to do.You might even borrow a set of taller tires for a while and see if they improve your fuel economy enough to be worth buying a set that size.
Just some possibilities to consider.
Speed
