Originally Posted by Monoborracho

+1 to the above EXCEPT, this would be off by a factor of 10 on my Cateye computers, and much more if your computer is in inches (83 x 25.4= 2108). I have Cateye Enduro 8's on three of my bikes so I am always resetting something. These three bike computers use a setting equivalent to the rolling circumference measured in centimeters.

I think Yen's computer is set in units equal to 1/10th of an inch. Then again, maybe it is set to use inches with mph and centimeters with km/hour. (1 inch = 2.54 centimeters)

You guys are working this too hard. You can calculate circumference with diamters and pie, or pigh, or pi, but Bill Kaupan above has it right.

Yen, your tire is a 700 x 40. The instructions should list a setting for this tire. If they do not, go to a smooth floor, set the inflation valve on the bottom, and mark the spot. Roll it one revolution in a smooth straight line. Measure the distance on the floor in inches and convert to centimeters (cm) by multiplying time 2.54. That is the most accurate way to set the computer, using the actual rolling cirucmferenc. A 700 tire is about 83 0r 84 inches around, which would make a setting of 210 cm to 213 cm. Cateye lists a circumference of 220 CM for a "standard" 700x40 bike tire, so your circumference should be about 86.6 inches.

But you have a default setting of 820? It might mean 82.0 inches. Maybe your computer uses 1/10th of an inch as the calibration, in which case 820 = 82 inches = 209.92 centimeters (go ahead and call it 210) which is also "standard" circumference listed by Cateye for a 26" x 1.5" tire, a mountain bike size.

The hardest part is finding your own language on the little folded up manual, and then finding it again when you need. Why do I buy the same computer? So I can have extra copies of the little folded up instructions.