The short answer is the RPM that the engine makes its peak torque is the RPM, or just faster, that you want to run at when cruising on the highway in high gear. This makes for a drivable vehicle, and good fuel economy.
Several things factor into this calculation, including HP and torque available (what engine). Also what does the truck weigh, and how much air does it push out of the way? And how fast do you cruise, 95 mph needs a fair amount more HP, and therefore more RPM than 55 mph does.
These last few things are hard to put a value on, but you can learn from others. If someone else has a similar setup, has it geared reasonably well, and is pleased with it, plenty of get up and go, and decent economy, then you are pretty safe with that engne and vehicle to calculate for the peak torque rpm at cruising speed, as this is near ideal in most cases.
If you don't have the HP, have a heavy vehicle, and/or drive really fast, then you will have to compromise on one of those variables, or run more RPM and sacrifice fuel economy.
This is a good Tech tip:
http://www.stovebolt.com/techtips/calculate_rpm_tire_axle.htmAnd I really like this calculator for figuring speeds and RPM.
http://www.onlineconversion.com/bigger_tires.htmGrigg
