This morning I checked my ClearOut bike from some BOs ago. Its tyres are labeled 42-622 and 28x1,75 which would not be skinny anymore. The example I brought up earlier was 40-622 and 28x1,5 which would be the max to be considered as skinny so far. A few minutes ago I checked my daily ride the Wolverine, it runs on 35-622 front and 47-622 back, so front skinny yes, back nope...
If I would take out my caliper gauge to measure the real thickness of the tyres I most probably would get another set of numbers and it would get complicated.
I know, that the exact conversion inbetween measures is somewhat off. So I would suggest we stick to what the tyres are generally labeled and not start measuring and converting back and forth...
So speaking for all 28 or 622 or 700c tyres (which are most of the tyres I have around here):
If its above 40/1,5 it's not skinny anymore anything labeled with this numbers or lower is fine.
For lower diameters (27/26/24/20/18/...) I would suggest a similar approach but I'm lacking examples/experience there...
Maybe we could set in general that the labeled tyre width shouldn't be above 40/1,5 no matter what diameter.
Is that something we all could agree on?