Odd tire size?
 

I was asked to look at a friend's old Kona MTB as he needed new tires. The tire size listed is 28x2.09. I couldn't find that size anywhere. Has anyone seen this size? Is there another that I can put on?

Where's the facepalm emoji when you need one. Completely forgot to google it. :foo:

Look for the ETRTO tire size designation on the sidewall. It should look something like this: 53-622

The first number is the nominal width of the tire. Rims can accept a range of widths, so it isn't strictly necessary to match this exactly, but unless you have a specific need for a different width, you should at least try to come close. The second number is the bead seat diameter, and this must match or your replacement tire won't fit on your rim.

+1 The ERTRO system, codified as ISO 5775, is a clear and unambiguous tire sizing system which is vastly simpler (IMO) to comprehend. https://en.wikipedia.org/wiki/ISO_5775 Compare this to the mishmash of sometimes bizarre and unintuitive systems it replaces, which had as many as 7 "26 inch" tire sizes as described in this article: https://www.sheldonbrown.com/tire-sizing.html

Here is a nice chart (part of the above article) which gives suggested guidance for tire width vs rim width: https://www.sheldonbrown.com/tire-sizing.html#width