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  1. #1
    Member ccrow's Avatar
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    Tracking Mileage

    So, i'm just getting started and I have an expensive ($20) computer on my bike. I thought I had it set up correctly, followed all the instructions, measured the circumference of my tires with me on the bike. I compared it's calculated distance against Strava (Iphone GPS app) and they don't match up. I think they were off 3/10 over a 10.5 mile ride.

    I'm sure I'm over scrutinizing at 3/10 difference in 10 miles, but I'm not sure which one would be correct.

    On the mathematical side, I'm not completely sure I have my tire circumference correct. When I compared my measurement to the suggested measurement in the instructions for 700x32 tires mine was a little less. I figured it was do to compression of me being on it. Admittedly I don't know if a few mm's would be throwing it off that much.
    05' Gary Fisher Nirvana

  2. #2
    Senior Member
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    Quote Originally Posted by ccrow View Post
    I'm sure I'm over scrutinizing at 3/10 difference in 10 miles, but I'm not sure which one would be correct.
    So the difference is 1 mile vs. 1.03 miles. I don't think that's a meaningful difference or one you need to worry about.

  3. #3
    SuperGimp TrojanHorse's Avatar
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    No, it would be 10.5 vs 10.2 which is a bit significant.

    Just go with your GPS app and use the computer for real time input while riding. Every now and then you'll get crazy results there too though, like a little quantum leap.

  4. #4
    Climbers Apprentice vesteroid's Avatar
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    It's a 3% difference any way you slice it.

    No more or less significant, just more or less miles different depending on the amount miles ridden.

  5. #5
    Senior Member volosong's Avatar
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    An easy way to measure a wheel is put the bike on a sidewalk with the valve stem in the bottom position on a crack. Roll the bike forward until the valve stem is again in the bottom position. Then just measure with a tape measure, (and convert to cm/mm if you don't have a metric tape). Change in distance due to tire deformation of a rider's weight should not be significant.
    Deut 6:5

    ---

    "Ha ha! You fool! You fell victim to one of the classic blunders - The most famous of which is 'never get involved in a land war in Asia'".
    - Vizzini during his "battle of wits" with the Man in Black

  6. #6
    Senior Member Seattle Forrest's Avatar
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    I don't think many people actually measure the circumference of their tire; it's usually a roll-out test where you put a drop of paint on the tire, then roll the wheel a few times, and take the average distance between spots on the road.

    Your bike computer is almost surely more accurate than GPS, just based on the way each system works.
    Don't believe everything you think.

  7. #7
    Senior Member
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    I use multiple apps and a computer on my bike and all give me different results, but within 1-1.5% of each other. Strava rounds up.

    I use Strava, Endomondo, and mapmyride(sometimes). Strava and endomondo tend to be closer together then my bike computer.

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