Originally Posted by
Robert A
Yes, that's how I measured the circumference of exactly 84" or 2133.6mm. The Bolt is calibrated to 2136mm, so that explains 0.2% of the 5% difference. The overread on the Bolt is rather consistent compared to GPS and by a fairly wide margin -- about 1 mph. Makes me question if the speed sensor is correctly calibrated.
You have to measure roll-out with the rider on the bike since the tires deform.
You want the circumference of the circle with the vertical radius from the hub to the road with the bike loaded. This radius is shorter than with the tire without any load.
GPS distance will tend to be lower than the distance measured by a correctly-calibrated wheel sensor. The measurement "error" (variability) is larger for GPS.