I just swapped out the tires on my bike from 700x23 to 700x32 for the rougher back roads and chipseal in this part of Texas.

I have the correct size tubes on the wheels now but all my spare tubes are the skinny 700 x 20-26 size tubes.

Are there any potential problems with carrying the smaller size tubes as spares? The only thing I can think of is that if the smaller size tube is inflated in a larger size tire the tube will be stretched thinner and would therefore be more susceptible to puncture (i.e. it's easier to pop a completely full balloon with a pin than a 1/2 full balloon).

Unless you are riding in thorns...who cares. I think the stretching would not be that much

generally it is not an issue but i have had the smaller tubes stretch so thin that the tube itself just failed mid ride.

Yeah, I know it will hold air until I get home. That's not really my worry.

My normal method of dealing with flats is that when i flat out on a ride I just swap tubes. Then when I get home I patch the punctured tube and put it back in my pack as the spare and leave the replacement tube in the tire.

My question isn't so much whether a too small tube will work in a pinch, but whether there are any reasons why I wouldn't want to just leave it on the bike permanently if/when I swap out tubes. Am I asking for trouble in any way?

Why ? a tube for 32 wide tires is not that much of a burden to carry is it?

Physics.. thinner wall stretched tubes lose air from seepage quicker.

And Goat head thorns are seeking out bicycle tires.

I wouldn't worry about it.

If its a brand new tube, but just smaller it should be ok.

If it's a patched tube, probably not... I've had patches come loose when the tube is too small and stretches too much.

Yeah well, I'll get the right tubes eventually. My LBS didn't have any 700x32 tubes with long valves in presta so I'll have to mail order some. The correct size tubes are on the bike now, I'm only talking about using the skinny tubes for spares during the interim.