Old 10-22-13, 04:37 PM
  #12  
gsa103
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Originally Posted by dragoscscc View Post
Wow, thanks for all the replies. Lots of things to chew over...

The tire is a Kenda Kwick Trax K1053-003 700x32C

The 2173mm I think I got it by rolling a piece of masking tape around the tire and then measuring it. I guess a roll out measurement would be more accurate or even measuring 10 miles distance or so...
Also, all the things mentioned make sense. Front tire goes a longer distance, subtle differences in route, etc.
Your tire compresses due to the weight of the rider. This results in the effective rolling diameter being less than the unloaded rolling diameter, hence a smaller circumference.

Basically, you have a couple of options:
1) Use a generic value from a table somewhere (usually gets within 2% for average riders)
OR
2) Dedicated calibration using a roll-out or straight road segment

Remember that this is a function of tire pressure and wear. The more accurate you want to measure, the more often you need to re-calibrate.

Also, don't assume that GPS will be more accurate. GPS is usually less accurate than a sensor, because the GPS clips corners. The faster you ride, the worse GPS tends to be.
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