Even stock OEM 214s you're going to be tucked in under the flares a bit. A couple other members here recommended I try the 20mm in the rear and it looks great in my opinion. 15mm for the front. It was quite a noticeable difference in person afterward. http://www.turnermotorsport.com/ (call)

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr