As you'll probably find in multiple threads - I don't believe it's so much the 20s themselves. On runflats, which is what you'll have from factory, they will be stiffer. If you don't mind not having runflats, you can get the 20s aftermarket and get your own rubber (Michelin Tour Latitude HP or Toyo Proxes S/T - both non-RFTs)... and the ride is much more soft - I felt an incredible difference especially noticeable when going over speed bumps.

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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