I live in Chicago and I run winter/summer setups on both my BMW's. For my 2012, I have 336's for my summers am running 211's (255/19's) with Pirelli Ice and Snow and they grip like crazy and stop extreamly well. The 18's for the E70 are 255's as well so you don't really gain any contact patch choosing a 18" rubber. I purchased the OEM 211's (off ebay) with OEM rubber delivered to my house for 1200.00 which replaced the rubber when I leased my 2007.

If you are running 20's in summer, I would recommend getting 19's and not 18's. I had 20's on my E53 and put 18's winters. I sold the 18' winter setup in less than a month and purchased 19's instead. I felt the 18's were too soft compared to the 20's I was used to. When I have the 19's the are a little bit softer but that sensation disappears after a few days and I really don't notice it. The only time I notice the softer tire again is when I put back on the 20's and the ride firms up.

People mention that 18's are better for pothole protection but I have not had a problem and I live in one of the pothole capitals of the world..

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2012 X5 50i ZAP(2VA, 336, 465), ZTP, ZPP, ZPS(6VC), ZCW, ZRC, 322, 4UB, 4NC, 610
2011 ///M3 Silverstone II
2011 X5 50i(lemon)
-- SOLD --
08 550 Msport
07 X5 4.8I sport
04 545i sport
02 4.4i sport
01 540I M/sport
00 SL500
97 SL500