Another option that would work in a pinch is to go ahead and use a 17" as a spare. It will work fine for the rear. If you get a flat in front, move a rear wheel to the front, and use the 17" spare in its place in the rear.

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2012 xDrive35d
2001 BMW X5 3.0i 5MT
and two 1987 Corvettes - Callaway Twin Turbo and Guldstrand Grand Sport 80
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