Avocet gives 2.7273 and explains that this is 30 / 11 (actually 2.7272727272...)
30/11 is easily recognized as closely related to the ratio of miles per hour to feet per second, 22/15.
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 calibration compare:
25.4 * 0.108 is 2.7432 -- slightly but significantly different.
2.7432 *11/30 is exactly 1.00584
There are exactly 1.609344 kilometers in a mile.
1.609344 / 1.600000 is also exactly 1.00584.
Therefore, the calibration numbers for kilometers will be slightly different (but also direct) because the computer uses the rounded factor of 1.6 when converting miles to kilometers. The error from not making this adjustment is 1/2 of 1 percent, which is significant, as bicycle computers can be calibrated by rollout to greater accuracy than this. The Avocet manual gives different calibration numbers for miles and kilometers, as you do.
What I'd say on the site is:
- For miles: circumference in inches x 2.7373 (11/30)
- For kilometers: circumference in millimeters x 0.108.
(Note: the two results are slightly different. Use the appropriate one for greatest accuracy)John Allen
