Consider measuring the actual wheel to confirm with him on the bike and tire inflated to riding pressure. I've taped a small string on the ground, start the valve core directly on the string and roll wheel one revolution and measure the distance.

If it is calibrated and functioning correctly, the cateye should be more accurate than a device relying upon GPS data.

---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

blueQuintana wrote:

650c

650c wheel circumference can vary greatly depending on tire brand, bike + rider's weight, and inflation pressure. As already mentioned, do a weighted wheel rollout to get an accurate circumference. I used two pieces of tape on the floor, one at the start of the revolution and one at the end, and used the Park Tool R-12 to measure the distance in millimeters.