Computer tyre circumference tables - why the difference?

wiggofan
wiggofan Posts: 30
edited April 2013 in Road buying advice
The Cateye circumference table says 2105mm for a 700x25c tyre, but the VDO table says 2146mm for the same tyre. Why the difference?
No longer a Wiggo fan

Comments

  • bompington
    bompington Posts: 7,674
    Dunno, but I would always actually measure it out myself anyway.
  • wiggofan
    wiggofan Posts: 30
    Yes, I think that's probably best.

    Another question. I've just received a new VDO computer, and the manual says it arrives with a battery installed but in despatch mode to reduce power consumption. However, mine arrived in 'sleep' mode, the clock had already been set, and the trip distance showed .02 and ave and max speed both showed 14.54. Does this mean it's been used or is this part of normal testing?
    No longer a Wiggo fan
  • sungod
    sungod Posts: 16,532
    tyres vary, there's no fixed circumference, assuming the sensor is on the front, the best way is to roll out the front wheel in a straight line on a smooth floor and measure the circumference

    if the packaging was all intact i'd assume it's just another case of user manual not matching reality
    my bike - faster than god's and twice as shiny
  • Crimmey
    Crimmey Posts: 207
    I measured mine with 25mm tyrrs and came out with 2120mm, just for comparison.
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  • wiggofan
    wiggofan Posts: 30
    sungod wrote:
    if the packaging was all intact i'd assume it's just another case of user manual not matching reality

    Thanks for the replies, really appreciated. Regarding the computer being in 'sleep mode' and not 'despatch' mode, you could well be right. All components were in sealed plastic bags except the computer and 2 transmitters, which press fit into cut outs on a cardboard backing. The main box was sealed, pristine and didn't look as if it had been tampered with.
    No longer a Wiggo fan
  • mikenetic
    mikenetic Posts: 486
    Tire carcasses measurements are based on width. Some don't have the same vertical profile i.e. some stand taller than others.

    If you want it really accurate you need to do it manually, with your weight on the bike so that they are compressed, which reduces the rolling diameter a bit.

    It's only affecting things by 1-2%, but then that could be the difference between a century or an unlucky 99-miler, I guess :D

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