Mine has always had the 275/35 20's. Yes, the running circumference is different, which translates into several things:
- the speedo should be more accurate. At least on W12's, the one data point available in the 18 inch fitment used in a German (language) road test is that the speedo was very optimistic--several miles per hour equivalent at around 60MPH/+/-100kph. The 20" fitment improves this since the tire diameter is 27.7", up from 27.0" on the 18" or 19" stock fitment and from 27.3 on the S8 style fitment with 265/35 20's. In the practical world, at least on mine using a GPS speedo app on my iPhone, it says it is essentially right on. It definitely does not read too low (the scary scenario for ticket risk).
- The overall diameter going up on the 20's in terms of the sidewall means when you do the math it is only .15" of sidewall rubber difference compared to the 19's as far as smoothness issues.
- The .7" in diameter change helps fill out the fender wells that much better, and I think the D3 looks better with the "big wheel" look--as do some other cars like the big Jags as another that comes to mind and also with big fender wells. If you VAG COM lower it you can decrease the fender gap incrementally more, yet the 275's increased the baseline ride height .35" to start with to help the undercarriage survive in the real world.
- And last, yes, the spare does not fit in 275/35-20 sizing. I've tried it. The running diameter is too big to start with, even really trying to wedge it there (good luck trying to get it out...). Plus I suspect even if I could jam it in there--letting a bunch of air out and a big mallet "persuader"--the width would make the trunk floor piece ride up. All why the 20" factory wheels like mine come with a 255/45-18 mounted on the (U.S.) baseline 18" five spoke wheel with a black paint job and some nanny safety stickers. The tire is a regular M+S Pirelli though, not any kind of mini-spare, inflatable or donut.
All this puts aside that I like the width appearance wise, and the handling feel and stop right now braking power. And meanwhile per the replies, agreed that as long as they aren't thought of as a real "snow tire," the Michelin AS3's are on my list when I burn off the current Dunlop Sport Maxx summers.