on decelber 12th, I tested a 21 inch Q7 wheel (10X21 ET44 with 20mm tailor made spacer) on the RS5 on the front then on the rear... each time, i made a little tour in a roundabout to check potential rubbing.... adn good news it doest rubb. there is more room in the front then in the rear very different than A5/S5 chassis...

On the rear its prety tight... here are some pics

a pic with front wheels mounted:
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same on the rear:
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To see how tight it is on the rear I pisitioned one side of the rear on a bump to haev it go inside the fender on that side by about 2 cm or 0.75 inches
here is the pic:
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