Just to be precise, an inner tube will not ignite from a spark from a ferro rod. (Thank God.) So, if you go out to the rain with only a ferro rod and an inner tube while everything else is wet, you will have major difficulties getting a flame going. However, an inner tube is good fuel if you already have a flame. (Please don't inhale the smoke.) This issue reminds me that it's important to practice beforehand and not to take any stage of fire starting for granted.