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I just bought some TPU Pirelli SmartTubes 700x23-32, and my road rims are 25.5mm inner width. My current Pirelli tubeless 30mm labelled tires measure out to 33-33.5mm actual width mounted and inflated.

I am going to use the TPU tubes with Continental GP5000 30mm tube-type tires. I understand that TPU tubes don't stretch much at all, and if you do stretch them too much they can be prone to flats. So, I'm wondering if it's OK to use 700x23-32 TPU tubes on the GP5000 tires, which if they are like the Pirellis will measure out to 33-34mm actual width. Another way to ask this is, are TPU tubes labelled to match the labelled tire width, or the actual measured tire width? Thoughts?

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  • I would assume that slightly exceeding the spec will be fine. I’ve used Schwalbe butyl tubes specced for 18–25/622 on 30mm (actual width, 28mm labeled width) tyres for years. I’ve only had one tube rupture along the seam. You’ll probably have more margin if your rim’s “valley” is fairly shallow, because then the tube has to fill less space.
    – Michael
    Commented Apr 2 at 8:32

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This is an understandable bit of confusion. The tube spec is given in terms of nominal width. That’s the width printed on the sidewall of the tire. You should generally assume that any specifications refer to nominal width unless the party takes pains to tell you that you should go and measure your tire. Few people do this. Silca’s tire pressure calculator does.

Generally, manufacturers designate their sizes by using a reference rim. A 25.5mm internal width is wide for road wheels. I don’t know what exactly the industry does, but the reference rim for those tubes, even though they are a very current product, is probably more like 21-23mm. That’s why your tires measure a bit larger than nominal.

Having your tires measure smaller than nominal is also possible but is rare. For example, I have nominal 28mm tires mounted to a set of rim brake wheels with a 19mm internal width. They measure about 27mm.

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