Greetings!

I have a Cateye Cordless. My instructions say that a standard 700X23C (what's on my bikes) is 2096 and that is the default size in the computer. For a 700X25C it says 2105 is the average size and for a 700X28C it is 2136. It further states that if that is not close enough you can set whatever you want and tells you to be sure the tires are inflated to the pressure you will ride at and that you sit on the bike and try to make it roll as straight as possible when checking the distance for one full revolution by the use of a piece of tape on the tire touching the pavement so that the marks on the pavement are placed as precisely as possible. This is because they assume that if the standard figure is not close enough for you, then you are pretty anal about accuracy! If you were really anal about it, you might ride the bike for a while to heat up the tires so that it is even more accurate when you measure it. Measure using a metric tape measure and you have the figure to set in your computer. Mine is set at 2098 currently (because I was curious how close the figures would be), but should be checked again because I bought new Gatorskin tires and have not checked it since then.

As others have said, there are many factors that will change or otherwise affect the readings for a given ride. Tire pressure, heat of the pavement affecting tire pressure, your weight, variables in how you make turns from one day to the next, how straight you ride, wear on your tires, wet pavement, the list is endless. What it boils down to is that it can't be that exact, but it is close enough for most purposes.

Regards,
Jefferito