Makes me wonder why to put in a full size spare and make any modifications to the car... How often you need a spare? Once in a lifetime maybe? And in any case, the spare one, even if same size with other wheels, will not be equally worn compared to all other three remaining wheels under the car so you will be going to replace it (or two tyres) anyway 
I've used my spare three times this year

Having a full size spare enables me to continue working, carrying passengers in safety with no restrictions on tyre speed, which in turn means no impact on journey times

Let's say you've got a cab booked to take you to the airport but you've only left the bare minimum of time to get there, ie 20mins for a 20 mile journey. I turn up on time and you're not quite ready, suddenly I've got 15 mins to drive 20 miles. You then get in the car and the first thing that I say is: "Good morning, just to let you know that I had a puncture last night and have a space saver spare fitted. It's perfectly safe but I can only drive at a maximum of 50mph".
How pisst off will you be and how stupid do I then look
