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I'm from a place where finding a standardized size for tyres bigger than 26 inches is mostly a hassle, to say the least..

But sadly (or not) I have a bike that I've been riding for a while now and have used it enough to wear out the tyres. The stock tyres on the bike were 700×35c. The rims on the bike are, as I mentioned above, 21-622 ETRTO. Now, the tyres that are bigger than 26' that are somewhat widely available around here and are ETRTO 635mm tyres. The confusion looms because, well, both of them are referred to as 28' tyres by more than just a handful of people..

It's simply used for commute on roads I won't particularly describe as best, and I avoid most bad potholes, if that helps.

So, if I'm unable to find 700c or 622mm tyres, it is safe for me to be using the 40-635 tyres without changing the whole wheel? I mean, I can probably find a tire that has a smaller width but a smaller diameter tyre seems to be hard to find around here..

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    It might work, but there is a serious probability that the new tire would pop off the rim. And I seriously find it hard to believe that you can't find 700C (xx-622) tires, as that is one of the most common sizes in use for road bikes.
    – Daniel R Hicks
    Commented Sep 15, 2020 at 12:13
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    Does this answer your question? How are tire sizes measured?
    – ojs
    Commented Sep 15, 2020 at 12:19
  • I've read the thread you're suggesting, ojs, but I still didn't understand whether I could put on a 635mm Tyre on a 622 mm rim, I was of the belief that "just" 15 mm worth difference isn't much, just a 2.5% worth difference and it won't be much of a problem if I try to put on the bigger tyre on..
    – Sim
    Commented Sep 15, 2020 at 12:28
  • @Daniel R Hicks, I come from a place where bicycles are looked at as a poor man's mode of transport and most people don't even know that there are specific bikes for specific purposes, let alone understand the tyre size standards! Bikes for them are categorised as "normal" and "geared" ; "normal" and "racing"... You get the drift.
    – Sim
    Commented Sep 15, 2020 at 12:35
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    "28 inch" is a horrid meaningless size. the ETRTO number is a confusion-free way of describing a tyre size, so don't feel bad to think of those riders as "old-fashioned" and take their words and advice with salt. Also, you have internet/web access, and there are plenty of web-based stores that will ship you stuff. Supporting the LBS is one thing, but not carrying common sizes makes them a poor LBS.
    – Criggie ♦
    Commented Sep 15, 2020 at 19:11

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The diameter measurement in an ETRTO specification is the rim bead set diameter in millimeters. You must use a tire with the same bead diameter as the rim of it will not fit. If you have a 622 rim you must use a 622 tire. A larger diameter tire will simply blow off the rim when you try to inflate the tube.

With regards to width, there is a maximum tire width depending on the width of the rims but the frame clearance is usually the limiting factor. See What is the maximum or minimum tire width I can fit on my bicycle.

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