I'm running 245/35R18 MPS4s on 18X8 et43 wheels and no spacers. No issues with clearance whatsoever. The xdrive is no different than the RWD in terms of fender and suspension wheel and tire clearances.
A 8.5" rim is not necessary to run 245/35R18s and most 8.5" rims for these cars have crappy offsets which then force you to run spacers and/or stretch or run narrower front tires.
https://www.2addicts.com/forums/show....php?t=1597122
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