The a6 tyre i was going to use is deflated and needs to be inflated before use,the same as the Q5 one.It has the same bolt centre's just wasnt sure how big the deflated tyre would measure both for the A6 and the Q5.Don't see how it can?
The deflatable spare fits the hole in the boot floor snuggly. By inflating it, it increases in size (obviously) to match the rolling circumference of the other road wheels. It cannot be re-fitted into the hole until it is properly deflated, which takes 2 hours.
If you take a wheel that will fit into the hole when inflated, you will have a spare of the wrong circumference. I'm no expert on differentials and drive train components, but I can't imagine that running an odd size wheel would be safe or good for the car?
As an aside, my pals A6 Le Mans has a full sized RS style 19" as a spare wheel - seems a much better way to go, although I did opt for the special spare and must say - it looks rather trick, bit like an F1 tyre in appearance?