I'd ensure that the load and speed rating is appropriate for the X5. Second, I'd ensure that the new tires can be installed safely on the wheel that you'll be using, that is, 255 to 235 may have different wheel width requirements. Other than that, I have no advice.

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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