Smaller spare correct?

[Wibbly]

I have 18" rims as the main tyres.

The compact spare is a 15" rim and the overall diameter (with the tyre fitted) is smaller, so the car will be 'down on one side' should the spare be fitted.

Is this a mistake on the part of Audi?

[Audi Owners Club.]

Maybe worth trying up alongside one of the wheels on the car to compare the overall diameter (with larger tyre on spare possibly?)

[Wibbly]

Overall size is significantly smaller. Audi dealer claim it's ok. Maybe I should ask another dealer?

Anyone else got a smaller spare than the tyres on their 18" rims? Same size? Perhaps not something you check until you have a puncture!? 

[Audi Owners Club.]

Seems strange that it would be a lot smaller diameter which would mean that the car would steer off in a particular direction because of the difference in ride height, Traction Control would invoke because of difference in rolling diameter, etc.  I would check with another dealer as you suggested