I always have a rain suit (rather than a poncho) in the car kit. Murphy's law says that a cold, sleeting rain will be coming down when they get a flat tire, and doing real work in a poncho is infuriating. A rain suit also acts as a windbreaker and traps the heat of other spare layers.

I use DriDucks or FroggToggs, which are cheap ($15-20), light, and pack down small.