275/35/20 on car, 255/45/18 spare.
 

Just checking if that is normal. According to Internet calculator it's 2% difference.
D3 4.2tdi sport.
Spare is stickered 50mph max.

Sounds like the spare might be an original wheel and the current wheels possibly upgraded to 20"
2% difference is right on the edge of what is acceptable, if not too much, I was always led to believe that 1% difference was the maximum recommended

Just copied this from an eBay advert-
Audi space saver spare wheel and tyre very rare rrp 889 plus vat with out a tyre

this is the space saver for vehicles with 20 inch alloys wheels fitted with a 255 45 18 dunlop tyre which is un used

this wheel has never been fitted but does have some wear marks from being stored in the boot

massive saving on new part number 4e0 601 025m

My D3 has an unused 18" spare, same tyre size as you stated, an ugly gloss black wheel?

Yes that's what I have.

Yeah mine has the same gloss black one that's smaller too, if I got a matching 20 inch alloy and tyre to what's on the car, would that fit in the wheel well? My a6 c6 avant had a full size matching alloy, so was surprised the S8 doesn't.

Pretty sure my 275/35/20 did fit in the well............just.
Got a niggling feeling though that I struggled to get the lid down completely flat.
I wasn't too bothered at the time, it was just short term until a puncture was repaired. So don't quote me on this. 98% sure though.

I'm sure mine is a 20" factory spare as anything smaller won't fit over my calipers.

Simple test, take one off and try +++

Good thinking batman, I'll do just that, if it does fit there's a matching alloy on ebay for £140, and I already have a spare tyre to put on that.