On Tuesday, October 2, 2018 at 6:10:14 PM UTC+1, Emanuel Berg wrote:
How wide tires can one put on a 622x15C rim?
Currently those on are the accursed 23-622
or 700x23C.

The ERTRO max is 2.5x the rim width over the bead retainer ridges, that is, not the outside of rim. So, supposing your rims are actually 15mm on the inside, you could theoretically fit 37mm tyres. If your rim manufacturer misleadingly tells you the outside measure of the rim, you should measure the inside width yourself and proceed from there.

Here's a worked calculation for a true 29er:
1. 1in + 736.6mm (we use a period, a full stop, for the decimal separator)
2. Rim diameter 622mm
3. (737 - 622)mm/2 = 57.5mm tyre width
4. Most desirable tyre for application is Schwalbe Big Apple
5. Schwalbe offers the Big Apple in 60mm, slightly larger than 29er requires, and the bike has clearances for such a big tyre plus the associated mudguards
6. Minimum inside rim width is 60mm/2.5 = 24mm
7. 24mm and 25mm inside measurement 622mm diameter rims are available
8. It may be better to use a rim width factor of 2, so 60mm/2 s = 30mm
9. 30mm inside width rims are available, built for monocycles

Andre Jute
So many choices. I blame Adam Smith.