The Style 209 is ET 46 (or 45?). I'm saying a 20mm spacer MINIMUM is going to be required. I don't really know. I don't know what tire you're trying to use, where they're hitting, or how hard they're fouling so I can't even say for sure what spacer you're actually going to need. Only that I'm guessing you have to get down to an effective ET 25 MINIMUM to have any hope of the new tires fitting. 20mm also happens to be the thinnest spacer that is still hub-centric. As a reminder, you're also going to need them to have a 74.1mm hub diameter which is not the most common BMW diameter. I think. I think it was only the E71 X6 that had mixed axles and hub diameters. Truthfully, I don't think you're hopes of a better ride will be realized with these tires and the required spacers.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim