I've calibrated both bikes with 25c tires using roll out method (with me on bike and tires freshly topped off to my chosen psi)

Both bikes matched my commute distance to .01mi. 6mo later I started using a Garmin GPS and it matches 2-50mi rides to .02mi.

My 8.48mi commute never varies by more than .01 mi. When it does it is because I've done a different 'thing' such as a very wide turn, say for construction, where I normally do a narrow (that changes by .05mi) or if my tire pressure is going down, which adds .02mi. I've done the exact same commute route for several hundreds of times and these patterns hold up.

I weight 185lbs, my front Conti Gatorskin 25c at 110psi is set at 2089 circumference. The Cateye manual recommended higher if I remember right, but it was wrong.

Al