i think you should be able to downsize to 18 and still fit, but i really noticed no difference in ride comfort in going to the 19" from 18". what reason do you have to downsize other than cost of tire? and don't forget where you live. if it clears up where you are, it's nice to have a high performance winter tire that can take the speed and has decent traction and stability. i went with dunlop M3 wintersport v rated for winter. outstanding tire with some noise, but the traction was incredible and i was still able to drive 140mph when the roads were clear. and for summer, since it is always raining in northern europe, i went with goodyear f1 g3. i only hear wind noise until about 85mph and they stick to the road so well if you have a slant in your lane you actually have to counter a little or they will travel a straight path- straight into the ditch.
Cost is secondary to performance to me. I prefer a narrow winter tire with more sidewall for best snow performance, so that's what I'm hoping for. A 17" setup is a great compromise for my winter climate, but I seriously doubt the brkaes will clear. I ran Dunlop WinterSport M3'son 18" wheels (with 355mm Brembos) and had the goodyears f1's on a set of 19" BBS RG-R's for my M5. So we think a lot alike. But wow, those 19 inchers rode like h-e-double hockey sticks on Michigan roads. I'm sure to take a beating in this new RS4, but it'll be pleasant torture, right?!
the roads in germany are a lot like wisconsin, a lot like michigan. if the rs is like the s, the ride comfort is reasonable for such a car and the cusion from the tire won't much matter. if you are in question, put the winter set up on your stock wheels. if you don't like it, it's only one season so re-mounting your summers won't dink up the rims if it's just a one time good deal. i was skeptical about staying with 18" for the winters when in know 17" would fit the s4, and also when i went up to 19" for summers, but as i said before, i feel no difference in comfort, only performance.
Thanks Cheese for the pics of the German Sprint Blue RS4. That car has P-Zeros on 18" RS6 wheels over those monster brakes, so it appears that an 18" winter wheel/tire setup is do-able.
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RE: Ordered an RS4, have some Q's - 
