Originally Posted by shwnrttr
you can get those bike speedos pretty accurate if you measure the hieght of the tire(in inches) then multiply by pi(3.14) then convert the numbers to milimeters. this would be the value you enter into the speedo. that will get you pretty close to correct. but not perfect. my tire measured 2257mm. but to get a more accurate reading i had to enter 2233mm. into the speedo.
Another method for accurate calibration is to make a mark on the front tire where it meets the floor and a corresponding mark on the floor. Roll the bike forward (must be a straight line) for one wheel revolution then mark the floor again where the tire mark is. Measure the distance from floor mark to floor mark and convert to metric if you don't have a metric tape. This is the number in milimeters that you enter into the calibration.
Example: 2073 is the ball park number most speedometers suggest for a 26" wheel. Due to differences in tires that number might not be exact. Using the method I described above will be accurate for your tire diameter.