OK, I know I'm reviving an old threat here but I've been reading through the old threads learning about tandems and I came across this.

My experience is that sure you can use smaller tubes in larger tires as many have already noted. HOWEVER....if you are old school and like to patch your tubes instead of just replacing them you will find that your patches will have a much higher failure rate if you are not using the correct size tube.

Tubes are very stretchy and small tubes can fill the space of a large tire. However patches generally do not stretch much at all. When you patch a tube you are affixing the patch to the tube when it is in its relaxed (un-stretched) state. When you then put the patched tube in a larger size tire and inflate the tube will stretch to fill the tire but the patch will not which will put a LOT of sheer force on the patch adhesive. If you are obsessive about how you apply patches you may get away with it just fine. But if you are using the new adhesive-less patches or are just sloppy about applying patches you are asking for failure.

Bottom line? If you carry around new replacement tubes then there's probably no problem using undersized tubes. However if you carry a patch kit and like to patch your tubes then you are increasing your chance of failure if you don't use the correct size tube.