The trend for car manufacturers to supply a can of gunk and 12v tyre inflator IMHO is a retrograde step for several reasons (as are run-flat tyres).
Not all tyre issues are ones that a can of gunk can solve. Hit a piece of discarded metal on the road (as recently happened to me with my C3) and the subsequent immediate deflation and a can of gunk will be unlikely to help you.
Even if you have a conventional puncture and use the gunk its highly likely that a tyre specialist will refuse to repair the tyre (due to all the gunk inside the tyre).
Much the same applies for run-flats - and get a puncture less than 50 or so miles from home - then subsequently visit your tyre specialist and he will most likely refuse to repair the run-flat, not taking responsibility or your word for how many miles you've use the tyre for and what potential damage has caused.
The only real advantage is that you can legally drive the vehicle with on deflated tyre (up to tyre manufacturers prescribed distance) if it is a runflat.........decide to drive-on a deflated standard tyre (and accept that you'll irrepaireably shred it) and get stopped by the old bill......and you'll get a nick.
I'm running low profile 18" tyres on my C3. These are very close in diameter to the OEM 15" tyres. I carry an old BFG as a spare. Even that is a little tight for the spare wheel carrier.....but it fits (just). My frontal puncture was on a concrete surfaced 'A' road. Some of these surfaces create a drone within the cabin, the road surface was poor anyway and I simply didn't realise I was running on a flat untill I smelt the burning rubber of a totally destroyed tyre. A can of gunk wouldn't have helped. I'd have to call a breakdown service to have recovered the car home. The trusty old BFG in in the spare wheel carrier saved the day.
If your not using the space for anything else....why not use it? - 14" and 15" tyres are few and far between (except for American brands) - but why not simply buy a cheap van tyre that will fit the tyre carrier)
