I think your dealer is trying to pull one over on you. I changed my 20's recently to my 19's for winter all that has to be done is to reset the TPM through Idrive. All the wheel sizes have the same circumference the only difference I can see is rolling resistance with larger tires/wheels. However, the brakes rotors on the 4.8i are larger than the 3.0i. Call another dealer and ask them.

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2013 BMW X6M-current
2013 Audi S4-current
2007 BMW X5 4.8i (E70)-sold