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I've seen questions concerning tube size but I haven't found anything matching what I'm trying to accomplish. I want to buy a few tubes to keep around for trails-side emergencies when mountain biking.

My mountain bike setups (3 of them):

  1. 27.5, 27mm inner-width rims running up to 2.3" tires.
  2. 27.5, 30mm inner-width rims running up to 2.5" tires.
  3. 29er, 25mm inner-width rims running up to 2.35" tires.

So I'm aware that I can use as small as 26" tubes for all of these but how narrow is too narrow? Would 1.1"-1.3" rated tubes suffice? Keep in mind that I run tubeless and I just want to have a tube in my pack that will allow me to get back to my car if I run into more trouble than a tubeless repair kit can handle.

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    All tubes I've seen in the past 10 years at least are in a box with the size range printed on the box. You get a tube whose size range encompasses the tire you're using. You can probably find a tube that spans the first two sizes, but the third is quite a bit different in wheel diameter -- 622mm vs 584mm. – Daniel R Hicks Apr 5 '17 at 0:16
  • @DanielRHicks is right -- the first 27.5 tube I found online would cover both 2.3" and 2.5". – Chris H Apr 5 '17 at 7:58
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    In general, a standard tube will span a width difference of roughly 20%, based on the package numbers, and you can probably go a hair beyond the numbers with no great risk. "Fancy" tubes may be more restrictive. – Daniel R Hicks Apr 5 '17 at 11:44
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If you're going to buy a tube to carry, you'd be better off getting something that fits properly. You don't want to push the limits of your tube (e.g. burst it when you first inflate it) if you're a long walk from anything else, and the extra bulk/weight isn't much. Also fitting a tube at the side of the trail isn't easy especially in bad weather (and punctures wait for bad weather to catch you out). So I would buy one tube for both 27.5" bikes (e.g. a 2.2--2.5" width) and one for the 29", then carry the right one rather than wrestle with a 26". You've got 3 MTBs so cost isn't an issue.

A tube for 1.3" width in a 2.5" tyre is stretched to twice its intended size, so it will be half as thick. If you're also stretching it in the other dimension it's even worse. (All slightly approximate).

That said, tubes are very stretchy. I've seen 24" tubes in 26" tyres. I've run slightly too thin tubes for months on end. In an emergency you can get away with pretty much anything that you can get to fit. But if you're planning, plan to do it right.

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    You'll be fine getting a 27.5" x 2.3" tube for the first 2 -- lose a bit of durability using a smaller tube in a larger tire. But I wouldn't stretch it in the 29er. You know which bike you're taking out, so its easy to pick the right tube for the bike. I normally actually carry a few sizes of tubes, just in case if someone I'm riding with needs one. – Batman Apr 4 '17 at 19:42
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    @Batman you're right. But the first 27.5 tube I looked up online is available in 2.2-2.5 which is perfect for both. Without any experience of 27.5 I find I'm OK with a carefully chosen 26" tube to cover 3 bikes and a carefully chosen 700c for the hybrid including its (wider) winter tyres, so only have to keep stock of two sizes. – Chris H Apr 4 '17 at 19:47
  • Fair enough, that's probably the truly "correct" way to go about this. I've seen some others use 26" tubes on their 29" wheels when dealing with flats on the trails so I thought I'd try and drop the weight in my pack by picking the smallest 26" tube I think I could get away with. At the very least, I think I'll just go with some 27.5" tubes for all three. If I have to change the tube in my backpack depending on which bike I ride, I will inevitably forget to do so. – Lloyd McFarlin Apr 5 '17 at 23:51
  • How you carry your gear is a very personal preference but I like to keep a suitable tube attached to the bike. I currently use a small triangular bag where the top tube and down tube meet the head tube, but have used a little saddlebag or a top tube bag. They are all big enough for a few tools as well. – Chris H Apr 6 '17 at 5:47

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