That would be a bad assumption. I've never measured the circumference of the wheel and use the pre-programmed circumference instead. GPS is generally accurate within about 3 feet ( or is it 3 yards?) at any point of your ride. In the car it even knows which lane you're in. I'd vote for GPS always being right before my wheel magnet.
In any case, the HRM is pretty cool to have. I was doing loops of Prospect Park and know every up, down, bump, curve, sewer, depression, even the lane markings, and I know what my effort will feel like at every spot. It will be good to translate that so I can consistently see it when I'm not in the park. I'm not sure though that I always want to know.
Last edited by zacster; 10-26-13 at 07:51 PM.