I agree with J** that measuring the length of one complete wheel revolution while you are sitting on the bike also works very well to calibrate your computer. You can do this by yourself by putting a dab of something colored on your tire, such as lipstick, and then riding a few wheel revolutions on concrete, then measuring the distance between two marks left on the concrete by the lipstick, etc. Measure carefully in millimeters. Most computers on the market today will use this measurement as the calibration number that you enter into the unit, e.g. one of my bikes has front wheel circumference of 2107mm and 2107 is the computer calibration number. I have used this method before and it does get very close to the same calibration number that you get by riding a known distance--I had a difference of only 1-2mm by the two methods, which would be 0.05 to 0.1% difference or about 2.5 to 5 feet per mile (not much difference at all).