Pros and cons of using 20-25mm inner tube in a 32mm tire?
I use a 700x23 butyl inner tubes (I've tested Michelin, Pirelli and Kenda) in a 700x28 tyre and I've had no problems so far. But 700x20 one is 'too thin' in this set, so it can explode. It depends on the brand and you can't rely on the assumption that every inner tube will work in every tyre.