Calibrating a Cycling Computer
The standard wheel size settings in the directions that come with a cycling computer may not be exactly accurate for your bike. This wouldn't matter much on short rides, but on longer rides being off a few millimeters in wheel circumference can make a BIG difference in the miles your computer calculates!
To accurately find your wheel size, make sure your tires are inflated to the pressure you want. Then position your bike on a hard, smooth surface with the valve stem in the bottom most position of the tire's rotation. Mark a spot on the floor directly below the valve stem.
With your full weight on the bike roll forward until your tire makes one complete revolution. Again mark the spot directly below the valve stem, and measure the distance between the two marks. Multiply the number of inches by 25.4 to convert to millimeters. This is the wheel setting number that you should set your computer to.