from the following, and supported by eg. Schwalbe's website, why do size equivalents go like this? :
40-622 = 700x40
38-622 = 700x38
37-622 = 700x35
32-622 = 700x32
25-622 = 700x25

etc.. ie. why does only the 37-622 tire have a different width when in the 700x## sizing (French sizing) of 35mm..? It only came up because I had ordered some 700x35 tires, and had a momentary concern when the box was labelled 37-622. I thought I had a return possibly on my hands.

Bike tires often measure smaller than the listed size.

Don't overly worry about it.

I suspect my Schwalbe Marathon Dureme 40-622 = 700 X 40, are really closer to 700 X 38 in actual diameter.

Very few bike tires run true to nominal size.

I would view it more as a reasonable substitution than as an equivalence

But isn't width, width? Just curious that just for this one size, the values differ.. Nothing too serious, just an oddity I'd be curious if someone could explain.

Typo.

Sheldon Brown's site has a great article about tire sizes: Tire Sizing Systems

No..
Vittoria

Schwalbe

Michelin.. which actually has some additional width different sizing also in the 40 and 42-622 sizes.


A1penguin.. I've been to Sheldon's site, I'm pretty sure he says the width should be the same also.. though he highlited that 1 3/4 is not the same as 1.75

Like in "Shakespeare in Love"..."It's a mystery".

The ERTO is for the rim interface and is basically a legal requirement to ensure proper rim fitment. The 700c x 35mm size should properly be called "something vaguely resembling 35mm". It represents the typically tire width when mounted on a "standard" rim for that size tire. For whatever reason, Schwalbe recommends the 35mm tire mounted on a slightly wider rim.

ISO (x-622 etc) is the correct way. 700xY is the blast from the past. Just like we often say 28" wheels for wheels with rim tyre bead diameter of 622. Just like we say 29" wheels, for the same rim width, just with a fatter tyre mounted on it.

All explained in detail, with cross reference of old standards:

Tyre dimensions

Thanks! Makes sense and that was my guess.. that somehow width measurement in ETRTO system was contingent upon some sort of assumed rim width, while the 700x?? system maybe not so. That said, even this link offers maybe some outdated into.. ie suggesting that even a 25mm tire is incompatible with a 19mm rim width (internal), when the flavor of the month seems to be to run 21mm(25mm external width) with 23 to 25mm tires. Their chart seems to imply that to run a Hed Ardennes rim, you need a minimum of a 35mm tire.

ETRTO is based on actual rim diameter. Unlike other, older standards.

Chart on rim widts is conservative - on the safe side. If one really knows what they're doing, it can be exceeded. Some rim producers, like Mavic for example, note on their rims what minimum and maximum width tyre rim takes, and what are maximum pressures it can take - the widest tyre having the lowest pressure allowed for the rim to hold.