Space saver spare wheel for S8?

[Greg66]

Apologies for the sudden barrage of questions - I've had an S8 looked at by the RAC and it has thrown up an issue with the tyres, and also the spare.

The report tells me that the tyres on the car are shod with 265/35/R20 tyres, as they should be.

But the spare is said to be a 255/45/R18.

I thought running different tyre threads by 4mm or so across the same axle was supposed to play havoc with the Quattro diff. By my sums, the spare will 6mm shorter than a regular wheel.

Interestingly, my PDFs of the 2006 and 2008 brochures make no mention of the spare. But in the Mar 09 brochure, on page 8, it says that all A8s come with a full size spare, but then a footnote says that if you order 20” inch wheels you get a space saver spare. It doesn't mention the size of the space save though.

An amateur S8 review I have read on Pistonheads makes a point of saying that the spare is full size though.

Anyone care to look in their boot and tell me what they have?

TIA.