You can either measure that way (but use 3 or more rotations and get the average) but another way is you clear the odometer on both the GPS and your truck. Program your truck to a specific size and write that down (Very important step, you want to be 100% sure on the size you use). Drive at least 15 miles (the more miles the better) and divide the truck miles by the GPS miles. Then multiply that number by the tire size you programmed in.
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