Today I had my first encounter with the wildly variegated world of inner tubes / tires / wheels sizes. My wheel shows this, no info readable on the tire:

enter image description here

I already discovered that 28x1.5 and 28x1 1/2 are two completely different sizes, as I bought the former inner tube and I couldn't manage to fit it in (it was too large). Now my question is:

I found a shop that sell 40-635 (28x1 1/2) inner tubes. Are they the same as this 22x635 (28x1 1/2)? Can I buy it safely or the tire itself may play a role?

If they are, why does the first number change so much? I thought it was the width expressed in millimeters!

  • 1
    The width stamped on the rim is usually the rim width, not the tire width. The rim is generally about 40% narrower than the matching tire. Commented Mar 7, 2017 at 13:02
  • Ah! So in theory the 40-635 inner tube should fit? And in case I would like to replace the outer tire as well, would its measure still be 40-635, i.e. do inner tube and outer tire have the same measures? Thanks a lot for the comment!
    – UJIN
    Commented Mar 7, 2017 at 13:15
  • There is the problem that 28 inch tires are oddballs, and may not follow regular conventions. Commented Mar 7, 2017 at 13:19

1 Answer 1


The 22x635 is the rim telling you its own ISO designation, which is internal rim width x bead seat diameter in millimeters. The 28x1-1/2 in parentheses is an attempt to help make it a little bit clearer what tire goes on it, or maybe help out in the case of a tire, presumably an old one, that only has that size printed on it. (28x1-1/2 and similar numbers are nominal sizes, and they make tire sizing confusing in a number of ways.)

Presuming your tire has an inflated width of around 40mm (say 37-43mm), and also presuming the valve type is correct, the tube you are looking at is the right one. I'm pretty sure virtually all 635mm tires are around that width.

  • So there is no 1 to 1 correspondence between the ISO rim sizes and the ISO tire/inner tubes sizes?
    – UJIN
    Commented Mar 7, 2017 at 13:30
  • 1
    Mostly no, because most of the currently used rim sizes have a number of different width tires that can go on them. So generally speaking with bikes you're matching the tube diameter to the tire and the rim, which must be the same BSD (the 635 or other 3-digit number) and the width to just the tire. There are some rim sizes (BSDs) that are very focused on one width of tire, and 635 is one of them from what I've seen. Also there are some diameters that are close enough together that the same tubes are usually used for both, such as 622 (700C) and 630 (27"). Commented Mar 7, 2017 at 13:38
  • Thank you, you have all been very helpful! I'll buy the 40-635 ;)
    – UJIN
    Commented Mar 7, 2017 at 13:41
  • Plus tubes are quite forgiving especially on the width -- they're normally specified with a range of widths like 28--45mm (and this one for example even says it will fit 622 and 635 -- and 630 as in Nathan's comment)
    – Chris H
    Commented Mar 7, 2017 at 14:14

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