Ok, so here is the real answer to this question.

In order to run lower rpms at higher speeds, you have to effectively change your gear ratio, either by taller tires , ring and pinion, or trans or transfer case reduction. If you don't want your truck to be taller, then reduce your axle ratios. You can go from, say, 4:10s to 3:56s, but you will loose bottom end. You can remedy this by using a double down case for off road, but shifting low/high at speed is a bit of a no-no. So the real solution is to change your ratios to achieve lower rpm at higher speeds, and increase engine torque to maintain bottom end. A mild RV cam and header can significantly change your power curve, which would work well with taller gears. A supercharger is also a good option. Turbos are better for higher rpm, superchargers typically make better bottom end torque.

The idea that more power means less gas mileage is a myth. Consider this example. A half ton Chevy truck with an automatic overdrive and a V6 gets less mileage than the same truck with a V8 in mild hills, because the V6 lacks the torque to pull the hills in overdrive, so it runs higher rpm.

I live in Georgia at the base of the Appalachians. There is one particular hill I drive a lot, speed limit is 55. Stock, in order to maintain 55-60, I had to shift down to 4th. After a mild cam and header, it will pull it just fine in 5th.

So change your gears if you don't want to go tall, and add torque. <img src="/forums/images/graemlins/kewl.gif" alt="" />