I think this is an x-drive issue more than pre x-drive.

Original e53 had open diff planetary gear that should allow any speed front or back. The x-drive uses a variable clutch that can/will lock the front and back axles. I'm not sure what the limits are for differences in rolling circumference on the original.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)