I just went with the numbers that mached my tire size in the instructions.I have set a computor by measuring the distance the valve stem traveled in 1 revolution.Trying to keep the front wheel rolling in a straight line ain't for me.I don't think I ever rode with a group and our mileage matched.Some varied by miles on a 54 miler the other day.I like to see the distance,rpm and average speed,but try not to get to hung up about them.