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700 x 21c tyre size

I'm in the process of calibrating my new computer and the tyre size guide in the accompanying booklet doesn't have my tyres in (Specialized Mondo Pro 700 x 21c). I've searched online and found nothing of any help on Specialized or Polar's websites or on various other forums I've searched.

I understand the most accurate way is to sit on your bike and measure the distance of a full wheel rotation, but I'm not going to be able to do this before the weekend so if anyone can help in the short term until I'm reunited with my bike that would be appreciated.


  • djtvdjtv Posts: 28
    I guess it depends on how accurate you want it. You can just put in the same number for a similar sized tyre (ideally a Specialized one if you have the size of a similar tyre). This will get you within a few %.

    The best way only takes 2 minutes and involves the following:

    Line up the wheel so the valve is at the bottom of the wheel and adjacent to a known point on the ground (mark 1).

    Walk the bike (or stand alone wheel) forward for 1 complete turn (i.e. until the valve is back down towards the ground).

    Mark that point on the ground (mark 2)

    Get a tape measure and measure the distance between mark 1 and mark 2. I guess this will need to be in cm for the computer but it should be obvious.

    That is the number you input to the computer.

  • I have used a Polar on various makes and sizes of tyre and found them to vary quite a lot. I have always used the manual set up as described by djtv but would add that you sit on the bike rather than just walk it as your weight does make a slight difference. Also make sure your tyres are pumped up to race day pressure.
  • ...and as a further modification to that, I do three or four turns of the wheel along a line on the road to make sure I'm going straight and to get an average: obviously the wheel size won't be changing, but it helps reduce any measurement inaccuracies. Also, make a chalk mark on your tyre, as lining up the valve against the road can be a bit inaccurate.

    Having done all this, why do I always think that my computer is reading slow? Surely I'm going faster than that?

    Is 2100mm close enough to be getting on with?

  • As a ball-park figure it should be, thanks Bopomofo. Once I've got a chance to do it properly I'll get my chalk and tape measure out.
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