I have a really stupid question for you guys. I've changed the tire sizes on 3 different trucks with the same tires and gears yet all three trucks required a different value for the tire size. I measured the diameter of the tire (from ground to top of tire), all three were the same yet I had to put in varying values to get them right with a GPS.
Is there some super secret method or formula to changing tire sizes so I won't have to test drive the vehicles for 10 minutes to get it right?