Does that mean I can carry a smaller spare tube and save 3.2 grams? Bonus!
Technically, 29" tyres are just extra-fat 28" tires, the rim diametre is 622 mm for both tyres. Surprisingly, 27" tyres have a larger rim, 630 mm, than 29" tyres.
The other parameter limiting the use of the tube is the width of the tyre. As 27" tyres have a maximum size of 35-630, 29" tyres start at 50-622. Thus, the tube for 27"/28" tyres will be somewhat overly inflated (according to it's specification) in a 29" tyre.
https://www.schwalbe.com/media/29/9b/36/1711611433/faq_reifengroessen_28032024.pdf
Wtf how did that happen?
Is it like 5.56, 5.7, .22, and .223, all being 5.7mm diameter?
I think this bit of insanity is due to 27 referring to the outer diameter of the tire rather than the rim diameter. "27" appears to be a road bike standard as tire sizes are around 1 inch on that Schwalbe chart. If you have a 630mm rim and a 1.1 inch tire, 630 / 25.4 + (1.1 * 2) = 27.0 for the outer diameter.
On the other hand, "29 inch" is a mountain bike standard with a tire size ranging from 2 to 2.4 inches or so. For a 622mm rim and 2.2 inch tire you get 622 / 25.4 + (2.2 * 2) = 28.8 outer diameter.
If the OP has a "29" inch tire, that's a mountain bike. And I suspect their "27" inch tire bike is also a mountain bike, which is actually a "27.5" size which is a mountain bike standard, with a 584mm rim diameter. If that's the case they would have had to stretch the tube a bit to get it on the 622mm rim of the 29er, but at least that tube would be intended for a mountain bike sized tire, say 2.4 inches. Using a tube intended for a 1 inch road tire in a 2.4 inch 29er tire would be more of a stretch.
You can also cut up old tubes and use them as patches on other tubes if you only have glue left :)
Back before there were puncture protector readily available, we would use old tubes to cover our new tubes to give another layer of goathead protection.
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