I have been calibrating my Sigma BC1600 Computer after a battery change. As I have 22-622 Conti Force tyres, the size was not in the Wheel Size chart in the Manual, so I thought I'd go to the fantastic Sheldon Brown site http://www.sheldonbrown.com/cyclecomputer-calibration.html to find out how to calculate the Wheel size.

This says:

The I.S.O. tire size consists of a tire width and a bead seat diameter. Both of these numbers are in millimeters. For example, a 28-622 (700 x 28C) tire has a nominal width of 28mm on a rim with a bead seat diameter of 622 mm

To get an approximate diameter (in mm) add the bead seat diameter to twice the tire width (since the tire comes into the diameter twice: 622 + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference in mm = 2130.

Well the Sigma Maunual had the wheel size at 2149 for a 28-622, a difference of 19mm. I got suspicious, so I checked calculations for a 23-622 which is a pretty standard road bike tyre size. The Sheldon Brown calculation was 2098mm against the Sigma calculation of 2133mm a difference of 35mm. This is an error of 1.6%, which is quite a bit really . I checked the calculation by doing the roll out test to actually measure the wheel circumference and hey presto, the Sheldon Brown calculation is right. The Sigma Manual also suggests this roll out test, so they agree with the Sheldon Brown formula (which isn't rocket science, its simply the calculation of the circumference of a circle).

So, I'd suggest you check the Wheel Size you have entered into your computer. The roll out test is probably the most reliable, but the Sheldon Brown formula is also pretty easy.