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I got a Cannondale SM 400, not sure which year, hope you can see the image below. enter image description here

The original outer tire says 26 x 1.40, think it's a road bike tire not mountain bike, so i bought a Bell tire 26 x 1 3/8 from amazon https://www.amazon.com/gp/product/B0796B9KZF

My friend who is a very experienced biker, just cannot install the new tire, its diameter seems to be 2+ inches longer. So we want to know what happened, which model share we buy ? both tires are saying 26". Thanks !

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  • I'm sure this is a duplicate, but in essence, 26" isn't always the same thing. 26× decimal and 26× fraction aren't compatible. Full details from Sheldon Brown
    – Chris H
    May 13 at 19:50
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    Can you please re-read the sidewall of the old and new tyres? You're looking for a number in the format YY-5XX or maybe 5XX-YY where the three digit number is 559 or 590 or 597. This must match. The other number is the tyre's width in mm when mounted and that can vary a bit with no real effect. Not trying to be a metric fanboy but there's one metric system and a bunch of incompatible imperial/sae/standard systems for bike tyres
    – Criggie ♦
    May 13 at 23:53
  • You guys awesome, the new one is 37-590, the old one is 37-559. Now the question is where to buy 37-559 ? seems i cannot find in walmart or amazon yet.
    – user3552178
    May 14 at 3:45
  • @Criggie i cannot find 37-559, are you saying 35-559 is ok as well ?
    – user3552178
    May 14 at 3:48
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    @user3552178 the 559 part MUST match, you have some leeway on the other number. Check out bicycles.stackexchange.com/questions/52587 for an existing answer that shows all that. But in short 2mm difference should be fine. Hopefully the supplier will take back the wrong tyre in exchange for the correct size.
    – Criggie ♦
    May 14 at 6:40

1 Answer 1

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26" is more of a marketing number than an engineering one. It doesn't always mean the same thing.

In particular, Sheldon Brown's law of tyre sizing applies here:

If two tires are marked with sizes that are mathematically equal, but one is expressed as a decimal and the other as a fraction, these two tires will not be interchangeable.

Sheldon Brown's website has a table of tyre sizes giving more detail.

So 26 × 1 3/8" is not the same as 26 × 1.375".

This is because tyres used to be sized by the outer diameter, with the rim diameter a matter of compatibility.

Modern tyre sizing uses the bead seat diameter and width, in mm. Most 26" wheels are 559mm -- almost, but not quite all of those with a decimal width. 26" wheels with a fractional width can have a bead seat diameter from 571 to 597mm. The bead seat diameter must match.

So your 26×1 3/8 tyre is probably a 597mm BSD, but could be 590mm. Your bike almost certainly needs 559mm. In modern ISO or ETRTO, you're looking for something like 35-559

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  • I couldn't find a close enough dupe on 26" oddities
    – Chris H
    May 13 at 20:05
  • You guys awesome, the new one is 37-590, the old one is 37-559. Now the question is where to buy 37-559 ? seems i cannot find in walmart or amazon yet.
    – user3552178
    May 14 at 3:44
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    559 is more common, and you don't have to match the width exactly. Marketing often uses the old sizing. So 26x1.35 is common and worth a search, and the ETRTO sizing should be somewhere in the product description (35-559) online, printed on a label, and moulded into the tyre itself
    – Chris H
    May 14 at 6:20

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