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I've just bought new Schwalbe Johnny Watts 29x2.6" and after putting it on the rim, I am not happy (to say it very gently). It is 60 mm wide, not 65 mm as it should be.

I would like to learn something for future purchases—is there a reliable way to estimate fresh/new tire width without putting it on the rim? Something that would warn me about such difference as above.

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    If you haven't added sealant or ridden, then contact the shop and ask if you can return for a swap.
    – Criggie
    Nov 26, 2022 at 9:57
  • It depends on rim width. On road bikes the trend to ever wider rims in recent years has resulted in real width often being quite a bit wider than specified. A “25mm” tyre can easily end up around 28mm.
    – Michael
    Nov 26, 2022 at 11:49
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    Curious: how are you measuring? Are measuring at the widest point of the sidewall or the knobs?
    – Paul H
    Nov 26, 2022 at 21:00
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    I was going to say that narrower rims will produce narrower mounted tire widths. However, I think that 30mm is about average for current gen MTB rims.
    – Weiwen Ng
    Nov 26, 2022 at 21:32
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    @PaulH, without knobs it is 59.5 mm, with knobs it is 60.5 mm. Nov 27, 2022 at 9:47

2 Answers 2

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Curiously, Schwalbe make this statement:

Why are tires often narrower than the stated tire size?
In order to ensure that tires have sufficient frame clearance, tire manufacturers generally prefer to keep production closer to the lower end of the permitted tolerance (+/- 3 mm).
Carcass casing materials have become more and more sophisticated over time. That reduces the tire widening after the fitting.
Furthermore, inflation pressure also plays a major role. With maximum inflation pressure the tire becomes wider than with low pressure. And a recently fitted tire still widens over time. This can make a difference of 1 to 2 mm.
The tire width is measured at the widest point, i.e. outside the lug.

(From https://www.schwalbe.com/en/reifenmasse )

This could explain at least half of the difference you're seeing.


Secondly, the internal rim width plays a part in how wide the tyre can be once installed. Again, using Schwalbe's recommendations your 65mm tyre could go on a rim with a width from 21mm to 50mm, which is an enormous difference.
https://www.schwalbe.com/files/schwalbe/userupload/Images/FAQ/reifen_felgen_2020/Reifen_Felgenkombinationen_2020_EN.pdf
The difference is that the same tyre will be narrower when installed on a rim that is narrower.

I have not been able to find what rim internal width is used for their sizing, but my guess is that your rim is towards the narrower end of their specified range.


How to predict? You can't really, other than knowing about Schwalbe's predisposition toward slightly narrower, and how wide your rims are already.

You might gain a little by increasing the tyre pressure while stored. A tyre will slowly relax, acquire a set and potentially gain a little more width. Adjust to riding pressure before use though.

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    Many thanks for all those references, hah, cruel world, the best way would be to look for the reviews and by trial & error at it seems. Just for the record, I used 30 mm internal width rim (so more to narrower half of their range). Nov 26, 2022 at 10:13
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    It's been my experience with mountain tires --mostly Specialized 29ers and all flavors of 26ers--that the listed width is nearly always a few mm's shy. The 30mm Internal width of your rim should yield the 2.6 width at the tread IMHO. Of the outside lugs, some are spaced slightly narrower than the adjacent pair. This is a subtle difference so make sure u measure at the widest spread pair.
    – Jeff
    Nov 26, 2022 at 20:07
  • A 67 mm wide tyre after a year on a 50 mm wide rim might wreck the frame. While a 60 mm wide new tyre on a 30 mm rim will just be a bit more narrow than anticipated. It is prudent to err on the narrow side.
    – gschenk
    Nov 28, 2022 at 15:06
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    I may expand this to an answer, but I have a feeling that each company reports nominal tire sizes based on a reference rim width - that we don't actually know for sure, but probably has some relation to commonly used rim sizes. I think that the average MTB rim is about 30mm internal width, and the OP reported that this is their rim width.
    – Weiwen Ng
    Nov 28, 2022 at 23:38
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One that works for the tires I'm interested in (the tires where rolling resistance matters), is looking up that tire on https://bicyclerollingresistance.com/

They have advertised and measured widths, plus of course the all-important rolling resistance information.

However, your tire, Johnny Watts, isn't available there. Maybe because it's an MTB tire as opposed to a road tire?

As for shopping tires of certain width, unfortunately they are often too narrow. It's explained here: https://www.sheldonbrown.com/tire-sizing.html#dishonest

Basically, a tire that's narrower is lighter. So a cyclist who buys a tire of certain width always buys the lightestweight tire, right? You can be the exception: if you buy the heaviestweight tire you can find that's of a given (advertised) width, it's more likely you actually get what you pay for.

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