Not a very fit oriented question, but if you want to calibrate with the most accurate tire size then try Making a mark on the tire and the floor. Roll the wheel exactly 1 rotation, make another mark on the floor and measure the distance so you get a number (metric) like 2094mm. Your computer should be able to use this number for exact sizing.

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Retul Certified Master Fitter, FIST certified fitter, Owner of Hypercat Racing http://www.hypercat.com, and friend to the animals.

2110mm

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http://www.mybicyclefit.com

I think this would be the most exact way - considering that 700x23 doesn't always measure the same between manufacturers. Plus I believe the rim that it is mounted to would affect the measurement as well. i.e. a wider rim will use more of the tires width and leave it 'shorter', making for a smaller circumference. I'm just grasping at this conceptually, I'm sure some math whiz can come along and back me up on this.... until then, go along with the data from the above method.

+1 with measure 1 revolution. Why guess when you can determine?

I use 2096 mm for this size tire.