Garmin wheel size , auto or manual setting ?

bing gordon
bing gordon Posts: 662
edited November 2015 in Workshop
Just wondering , do you set your Garmin to Auto or Manual wheel size ?. I've changed over to 700 x 25's and I'm wondering which is the most accurate option. I've measured my 700x25's at 2130mm. Also, do you take any millimeters off for tyre compression on the road ?

Comments

  • wongataa
    wongataa Posts: 1,001
    [url=http://www.bikeradar.com/forums/viewtopic.php?p=19735779#p19735779]bing gordon[/url] wrote:
    Just wondering , do you set your Garmin to Auto or Manual wheel size ?. I've changed over to 700 x 25's and I'm wondering which is the most accurate option. I've measured my 700x25's at 2130mm. Also, do you take any millimeters off for tyre compression on the road ?
    I have mine in the manual setting. You do have to compensate for tyre compression. You should measure the wheel circumference with the bike suitably loaded by a rider.

    Accuracy of the manual setting will depend on how well you can measure. The best way would be to start with the tyre valve at the bottom of the wheel. Mark the location of the valve on the wheel. With a suitable load on the bike roll forward until the valve is back at the bottom and mark the position. Measure the distance. To minimise measurement error do as many revolutions as you can reasonably do and divide the measured distance by the number of revolutions.

    The auto setting accuracy will depend on the GPS accuracy when the unit decides to work it out. Also the Garmin will keep tweaking the number with the auto setting. If you keep checking what it has been set to you will see the value changes.
  • bing gordon
    bing gordon Posts: 662
    Nice one thanks for that
  • buckmulligan
    buckmulligan Posts: 1,031
    I always go for manual just because the auto-function seems a bit all over the place. Measure the wheel size as bing gordon suggests, at your usual tyre pressures and with your weight on the bike to account for tyre deflection.

    If you have a regular route that you know the distance of (e.g. a commute) then you can always turn the GPS off for one trip and check that the distance is close and thus that your wheel size is accurate. TBH a few mm on your wheel size isn't going to be the end of the world; +/-10mm would be around +/-0.5% which is likely better than GPS accuracy in most circumstances.

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