Tire with no ETRTO/ISO size I'm contemplating replacing the tires on my mountain bike with smoother tires since I am rarely ever on anything other than paved surfaces. I read up on sizing and am aware of the xx-xxx ETRTO/ISO sizing that should be on the sidewall but I don't see it anywhere. Could only find inflation pressure range and some manufacturer's marks. They are Kenda. I could see K-816-015 on the front and K-816-027 on the rear. The tires have the basic size 26x2.1. The rim has RIGIDA 1.75 stamped on it. AFAIK everything on the bike is original (I've had it since new from 1994). Inner rim width is 21-22 mm.

Based on Sheldon Brown's site and also having googled Rigida 1.75 it seems to point my rim/tire being based on 559 mm bead seat diameter.

So, the questions am I right about 559 mm, and (2) for a smooth wall tire I was thinking of 1.75", but could I go to 1.5"?
Older tires may not have the ETRTO/ISO size markings. On your tires the '26x2.1' marking denotes rim diameter and tire width.


'26 inch' mountain bike tires have 559mm rim diameter.
2.1 inch is the nominal width. 


With an 21-22mm inner rim width you could run a 1.5 inch / 38mm tire, but probably not much narrower.