From my experience with the Allroad, 18" wheels with 40 section tyres are very harsh-riding. I guess if you're 18, it would probably be worth it for the better turn-in and responsiveness, but once you start to get irritated by crashing into potholes it's a one-way street, you can never get un-irritated with it! AND our bloody roads are full of potholes these days, despite the outrageous cost of road tax....don't get me started.
I have to say that coli-sprung A6's may be better than the Air-sprung ones, but I've never been in one, so wouldn't know. Audi's record in making smooth-riding cars is not a good one so far...
I've gone from 18" wheels with 245/40's, back to standard 17" ones with 225/55's. The difference suits this old fart just fine, and the turn-in is not bad at all, just not as good as it was. What I don't get anymore is this feeling of ultra-sharp turn-in, followed by washing into understeer......that's just tedious and a bit depressing. It ruins the balance of the car, making it all-but impossible to get the back-end to contribute to the turning process The quattro system then ends up being no better than FWD.....not what I took the ~15% fuel consumption hit for!
Never being able to leave things alone, I'm planning of upping the size of my 17" tryes to get some more grip, and more importantly, some protection of my precious alloys from the kerbs. I recon 255/50 will give me fatter sidewalls for protection, but keep the rolling radius the same (ish) to maintain what little accuracy the speedo has.
If you want to play tunes with tyre/wheel/offset sizes, this link is a good place to check if it's likely to fit under the arches, or rub on the suspension uprights..
http://www.rimsntires.com/specs.jsp
If you want change the offset, make sure that the imaginary line drawn through the steering axis hits the road roughly in the middle of the tyre width. If you stray too far from this ideal, you will make the steering wheel into a thumb-breaker!
