^ What they said.
Just make sure you click "Load Previous settings" so that nothing else gets messed up.
If you have pegasus that has a good tire size calculator in there. It's hidden in the gear ratio calculator, that'll help you get it close. If you have a GPS then you can get it perfect. Write down the tire size that you put in there then clear the odometer on the truck and GPS, drive at least 10 miles and then take the numbers and do this:
Truck odometer divided by GPS odometer multiply that by the tire size you were using. That'll have you dialed in perfect in no time.
