even if the ID of the 18" wheel fits, you have to make sure the Offset value is low enough to allow the street side of the caliper to have clearance against the backside of the wheel's spokes.
that can be just as tricky finding the right low offset as it is nice looking/quality wheel.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S