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10-22-13, 04:53 PM
#13
Keepin it Wheel

Join Date: Aug 2011
Location: San Diego
Posts: 8,611

Bikes: Surly CrossCheck, Moto Fantom29 ProSL hardtail

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Originally Posted by dragoscscc
Recently took a trip around town, the bike computer registered 100.7 miles. Looked it up on google maps and the same trip was 98.8 miles. Right now I have the tire circumference set at 2173mm. To correct the error, the new tire circumference would have to be set at 2132mm. (100.7/98.8=1.019230, in order to get the same ratio with the tire circumference it would have to be 2132) However 41mm difference is huge, I could understand a 4 or 10 mm error but 41mm per revolution? I remember, to get the original tire circumference of 2173mm I actually measured the tire. I doubt I was off by that much (1.6"!)

So clearly my math must be wrong somewhere.

Or there's a 2% error between real world and google...

Interested to hear some ideas

Cheers
D
All interesting stuff. I agree with some that it doesn't really matter a whole lot, but I'm a bit OCD and would feel compelled to "get it right".

I would recommend searching out a road with tenth-of-mile markings, coast "exactly" 0.1mile (choose the downhill direction) as arrow-straight as possible, and you will have accounted for tire deformation and wandering route as much as possible. Then recalibrate your computer's circumference using the adjusted calibration technique you describe, and you can sleep well at night.