Where do you put the flat tyre?

[PAH]

Boxster is in the car park, when a friend comes into my office to tell me I have a flat trye. Go out, change the wheel for the space-saver, and look angrily at the nail embedded right at the edge of the tyre - no repairing that. However, not too upset because there was only about 3mm left and then needed changing anyway.

Tidy up, and go to put the 18" Turbo Twist wheel and tyre back in the car. However.....it will not fit! I can't see any way in the world this will fit back in the car, short of putting it on the passenger seat. Not ideal - dirty, wet tyre and wheel covered in brake dust on my nice leather seats - I don't think so!

Any ideas? This seems mad - what if I'd been out with the wife in the middle of nowhere - do you have to hide the wheel in a hedge and come back and get it with another car???