Originally Posted by
mstateglfr
Itll work.
With that said, there is a limit. A 23mm tube in a 40mm tire is dumb and not safe as the tube isnt made to expand that much.
Have you tried pumping up a 23mm tube outside a tyre?
I can guarantee the diameter it gets to is more than 40mm, and it's nowhere near bursting. Think balloon. It might be slightly less durable, but still usable.
I've used 23mm tubes in 32mm tyres before now. No problems.