Help - Cycle Computer

Do a roll out James. Take your bike to a good flat cement surface. Draw a line. Place one of the tires directly on top of the line with the valve stem in the six o'clock position. Roll the bike perpendicular to the first line until the valve stem is in the six o'clock position again, i.e. one complete revolution. Draw a line where the tire ended up. Measure the distance between the first line and the second line. This is the circumferece of your tire.

This method is very accurate. All tires are a little different in their actual inflated size, so actually measuring the tire on the bike is much better than trying to figure it out mathmatically.

kdelong is right. You should also make sure the tires are properly inflated and put a little of your wieght on the bike to simulate the compression on the tires when you're actually riding. It is only slightly more acurate, though.