Originally Posted by
crazzywolfie
i have not not got to measure the circumference of the tires yet but i think it is 1km/h slow for very 4km/h so instead of my top speed being 50.4 it would be more like 60.4km/h. i noticed when i was walking with my bike today that i was only going 4km/h and the first bike use to go about 5km/h when i was walking with it.
This kind of thumbnail math can reduce the error from 25% to ~10% or so, which still isn't very accurate. Your bike computer is capable of accuracy down to about 0.5% or better.
The best way that I've found to calibrate is to ride a short path of known distance, and then do the math. GPS works, or Google Maps, or gmap-pedometer.com, or bikely, or whatever. Pick a route a mile or so long. Look close at the plotted route to make sure you're biking exactly that. Ride it and note the distance on your bike computer. Then:
(new calibration number) = (current calibration number) * (gmap or gps distance) / (recorded distance)