If calibrating these Avocet computers using a rollout test:

• For miles: use circumference in inches x 2.7273 (11/30)
• 
  
• For kilometers: circumference in millimeters x 0.108.

(The two results are slightly different. Use the appropriate result for greatest accuracy).

Why? Warning: math ahead!

Avocet gives 2.7273 for miles, and explains that this is 30 / 11 (actually 2.7272727272...)

30/11 is closely related to the ratio of miles per hour to feet per second, 15/22.

These Avocet computers are unusual, then, in being calibrated directly to miles rather than to miles by reference to kilometers.

But for kilometers, the same computers ask for the circumference in millimeters x 0.108.

(The numbers are from an original Avocet 20 manual.)

Now let's see how the mile and kilometer calibrations compare:

Therefore, the calibration numbers for kilometers will be slightly different, because the computer uses the rounded factor of 1.6 when converting miles to kilometers. The error if not making this adjustment is over 1/2 of 1 percent, which is significant, as bicycle computers can be calibrated by rollout to greater accuracy than this. The Avocet manual says to add 1 for kilometers, but this is only an approximation.

John Allen