Putting this here rather than in the Knowledge section because it is only an observation not a question and not really linked to the other tyre width questions.
When I built up my bitza 650B tourer i used the 37-584 Schwalbe HS159 tyres that I had already for the tandem (which donated the rims). My mate Christian, 650B mecano and fanatic, remarked that they were no wider than the 30-584 tyres made for the Confrérie by Hutchinson (which are on all other counts a superior tarmac tyre). He has now supplied me with a pair of Hutchinsons and yesterday was the time to fit them, during which I took some measures. The rims are ZAC19s, which I think have an internal width of 19mm (or something very close). Having a 44-584 IRC tyre from a bicyclette in the garage I put that on and measured as well (it was a tyre that I had bought about 10 years ago in the local hyper and the synthetic gum walls have gone a bit inflexible).
The results: 37-584 Schwalbe = 34mm wide
30-584 Hutchinson = 32mm wide
44-584 IRC = 35mm wide
(which is handy to know since it means that I can source emergency spare tyres in supermarkets: they are a slightly tighter fit and may be a tiny bit higher judging by the difficulty squeezing the axle of the hub pasr the mech but they go).
Of course as a good Confrére I am keeping the Hutchinsons on. I suspect that as well as being half the weight of the Schwalbes, the the thinner sidewalls mean a bigger volume of air and a bit more comfort