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We had to learn this the hard way in the GT-R world. Many manufacturers and tire size calculators that were "just right" still blew center differentials over and over.
A guy named Shep from ShepTrans (famous in the EVO/DSM/GT-R world for drivetrain) finally gave us the reality. Only way to know for sure is to install the damn tires, and put them on the car at running pressure, and mark them and do a slow rollout.
Unless they were REAL close, you'd eventually blow the center diff.
BMW's system is probably a little more tolerant. Who knows? I don't be the one to find out. I install either BMW "listed" tires and setups, or I install "square" (same tire all the way round) setups.
OP, reality is you can change the OD, but lots of unpredictables can occur as the result. Just giving you a suggested pathway to ensure safety.
Shawn
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