The 20" wheels that came w/my RS6 were just a little big and not wide enough (20x8.5 OZ Ultraleggara). After figuring out that a 2013 RS5 takes virtually the same size wheel it opened up my wheel choices on Tire Rack. I ended up choosing OZ Superturismo LM's 19x9.5 with a 30mm offset. All I needed to adapt them to the car was the 66.6 > 57.1 reducer rings. Ended up with a Sumitomo HTRZIII 275/30/19... fitment is near perfect with no spacers needed (also posted to the definitive wheel guide).