A 17 piece valve stem repair kit in a storage box.

Not every flat is a puncture. A leaking valve core, a stem damaged by a rock, or a core loosened by repeated airing down will empty a tire just as effectively — and none of them is fixable with a plug kit.

People who air down often are more exposed to this than most, because every deflator cycle works the core. A ten dollar kit turns that into a two-minute fix instead of a spare tire and a long drive home.

Small enough to live in the glovebox permanently.