If the rims are the same design front and back the only way to make the rims lighter and still have the OEM strength would be a harder alloy. I expect BMW would build the rims stronger than needed. I wouldn't expect the alloy on the lighter wheels to be a stronger alloy. I would be more concerned with the rear rims that are wider. That doesn't mean there will be any problems at all but---- Would be good to hear from others that have similar rims.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas