Who is running 265/35/20 tires? Any issues?
 

As the title says. Any issues with fender rubbing, etc. Looks like there should be enough room.

Plenty of room in there for 265's.

I assume that under "etc" you're ok with reading 1% slow on the speedo, throwing off the Nav, and whatever else comes from an off-sized tire?

What do larger tires do to the navigation?

Well to be fair I am a decade out of date, but the RNS-D for instance wants to know tire size for the "inertial" navigation - e.g. in a tunnel, other lost GPS lock times. It's not inertial of course, but it counts pulses from the wheel sensors to know how far you'd travelled. Running "wrong" sized tires would lead to some occasional oddities when approaching intersections (like "turn right ahead" when you were halfway through the intersection), noticeably longer "re-lock" times when returning to coverage areas, and less reliable startup directions. Admittedly one had to know what one was looking at to separate these from random GPS issues, but those of us who were doing the first US RNS-D installs were looking really closely at what was going on :D

I'm just now coming up to speed on MMI Touch, so I honestly have no idea if it matters to the C7 cars - but I do note Audi picked a pretty oddball 19" tire size (255/40-19) to match up to the 255/35-20 size they used on the 20" wheels. I tend to believe they did that to avoid more than just the slight speedo inaccuracy alone.

YMMV and all that, but my personal advice would be to stick with the stock size. As noted in my UHP A/S tire thread, I ordered 20" wheels on my car solely to keep a more common tire size, so FWIW I'm putting my money where my mouth is on this one :)

A 265/35/20 tire will only add 7mm overall diameter to the wheel/tire combo. Your tire diameter can change this much over time with treadwear alone. You really have to go with over 3% before you start to really notice any functional difference. A 7mm difference on a new tire is a 1% change. I am not worried one bit about the functionality, just the fit. FWIW, I ran 275/30/19 on my RS4 which was a 2% change (13.5mm smaller diameter) and I had no issue. Likewise from my experience with my M3 and the info on the M3 forums. No issue whatsoever with small variations in tire diameter sizing in terms of function.

No problem with the fit for sure. I'm running 15mm spacers on the stock wheels and there is no issue with rubbing at all. Plus, even without the spacers, there is tons of room on the inside as well.

Fair enough - I mostly wanted to make sure you'd been thinking about this, which it sounds like you have.

I confess to being mildly curious as to why you'd want to go ~4% wider on the tire though.

I'm running 265/35 on a 20x9 ET25 rim, which pushes the wheel 26mm outwards compared to stock. In roughly 13K miles I've rubbed twice and it was in the same turn on a big bump on the outside front wheel while the suspension was loaded. If your offset is less aggressive then you should have zero issues with rubbing.

For reference, this is with a Michelin Pilot Super Sport.

You lose nothing and more grip is more grip. For a car the size of the S6, I think a 255 is a little undersized. The S7 does, in fact, does use a 265/35/20. Not sure why they went with 255 on the S6. In fact, So far as I remember, the C6 S6 used a 265 tire as well. It also adds just a little more of an air cushion.