I bought a tube marked 26/27½″ for a tire marked “44-622 / 700×45C”. I have another bike w/a 26″ wheel. So despite struggling to trust a dual size tube to be good for either size, I thought it’ll be useful one way or another. But the immediate need is for the 700mm wheel (assuming the “700” means 700mm). That’s 27.56″, which is presumably the same as 27½″ in the world of “nominal” sizes.
The tube does not entirely pack into the tire and the innermost ring of the tube is exposed. Is that okay?
The previous tube was marked 28″. Not sure if that’s the correct size.
A 27 1/2" tube is not really the same as 700c. 622/700c is a pretty common size that has a bead seat diameter of 622mm. 27 1/2" is a rare Danish size with a bead seat diameter of 609mm. Tubes fit a range of sizes for a specific diameter tire; the 26" tube might fit the 26" tire, but you need a 700 x whatever width range covers your tire width for the 700c wheel. For example, Conti makes a 700x32-47c that fits a 700x45c tire.
Further complicating things, fractional tire/tubes are not the same as mathematically equivalent decimal sizes: 26x1.75" is not the same as 26x1 3/4".
If you want a really good explanation, check out Sheldon Brown.