Trouble is my BMW takes 19" 285 or something - they are huge in the boot - which is not that big to start with (Well not compared to the boot in my old Disco!)
To be fair, when off roading, I used to carry 2 spares - one underneath and one on the roof. In 7 years only had 2 punctures - both when off road. About 170k miles and a few thousand off road