Will 40-635 tyres fit on a 21-622 rim? 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..
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.