Tyre sizes

[Mike Ayling]

I have just replaced the Paselas on my Mercury with Marathon Supremes.

My question is the box is marked 37-622, 28X1.40 and 700CX35C

Why is the 700CX35C and the 622 is X 37. I thought that they would be the same?

Mike

[Danneaux]

My guess is the 37x622 is the precise ISO/ETRTO size while the 700x35C is the approximate French designation. See: https://www.schwalbetires.com/tech_info/size_markings
At this link, Schwalbe states...

The ETRTO size specification 37-622 indicates the width of 37 mm and the tire inner diameter of 622 mm. This dimension is clear and allows a precise classification of the rim size.

A bit later in the page, they say...

French size markings (e.g. 700 x 35C) give the approximate tire outer diameter (700 mm) and width (35 mm). The letter at the end indicates the inner diameter of the tire. In this case, C stands for 622 mm.

It is useful to read the entire article at this link, but this explains the seeming mismatch in your Schwalbe box marking. Makers sometimes differ in their sizing conventions, as this useful article by Sheldon Brown states: https://www.sheldonbrown.com/tire-sizing.html

Just out of curiosity, what is their actual, inflated width of your tires when installed on your specific width rims, Mike? A rim that is wider or narrower than optimal can affect the width of a mounted tire to a degree.

I've also had tires "grow" a bit over time as the casing stretches, sometimes by as much as 2-3mm in actual width in the course of a week or so. For example, a pair of Panaracer Paselas I bought were labeled 700x37C, measured 35mm wide when first mounted, and "grew" to the stated 37mm after a week inflated on the rims. EDIT: I just checked and in my case the ISO/ETRTO and French markings on these Panaracer tires match: 37-622 molded into the tire and the sidewall label says 700x37C. I no longer have the Panaracer box to check against the tires, as you do with your Schwalbes.

Best,

Dan.

[Mike Ayling]

Thanks Dan,

Very informative.

I have just been out to the shed with my non electronic vernier caliper and the tyre width looks like 34mm.

It is time for another eye sight test however!

Mike

[martinf]

I have just been out to the shed with my non electronic vernier caliper and the tyre width looks like 34mm.
It is time for another eye sight test however!

I have found that true tyre width is often a few mm less than the stated ETRTO size, for example my 42x559 Marathon Supremes are closer to 37 in real life.

I suspect measured tyre width may also depend to some extent on rim width.

[John Saxby]

Interesting issue, Mike.  I have had the same questions about the 770 x 35mm (622-37) Supremes I ordered for my derailleur bike a few months ago.  Mine worked out to be just over 33 mm inflated -- would have preferred a little more, but these fit nicely under my 45mm mudguards.

A 2-3 mm nominal-vs-inflated width difference for Supremes also holds true for the 26 x 1.6's which I have on my Raven. There, the inflated actual width is a hair over 39 mm, compared with the ERTO of 559-42 and the 1.6 / 40.64. (The size of the difference depends on the variable starting point...)  I have different rims fore and aft on the Raven, but the inflated size of the tires is essentially the same.

[mickeg]

Some manufacturers also seem to have a narrower bias, I have some Conti tires that caliper at 25 but are labeled 28mm.  I suspect they wanted to claim a slightly lighter weight and shrunk the dimensions a bit.

[Andre Jute]

Another factor is the temperature of the air in the tyre. A tyre up to operating temperature may measure wider than one with ambient temparature air in the tube, and a really hot tyre (after a long run at speed, say) may measure wider still.

It seems to me that it would be valid for a manufacturer to measure dynamic, operating temperature width.