Originally Posted by Looigi

Or: Measure the actual circumference of the tire using the roll-out method. Stand the bike up on a smooth surface. Position the wheel with the sensor with the valve stem at the bottom. Mark the floor directly below the valve stem, roll the bike forward one wheel rotation and mark the floor when the valve is again at the bottom. Measure the distance between the marks to get the tire circumference. Weighting the bike as it would be while riding will improve the accuracy and yield a slightly lower number, roughly 1% lower.