Originally Posted by NYJayhawk

Ok, time to clarify:

Get a friend to help you. Sit on your bike. Mark the floor where one revolution of the wheel starts and stops (easy to figure out if you line up your valve stem at the bottom of your wheel before you start). Measure the distance.

That's the number you put in to your computer....after converting it to millimeters, of course (multiply inches by 25.4).

PS: I have a Polar CS100, and the numbers in the manual are off by a HUGE margin...enough that I'm convinced that there's a typo. If I remember correctly, there was a difference of close to 100mm for a 25c tire.