I have 15mm all around and also have front flares. I'm happy with the 15s but I think 20mm would be fine as well.

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2022 X5M Comp Dravit Grey/Full Silverstone
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2018 M5 Black/Full Silverstone
2015 X5M Donington Grey/Full Silverstone
2010 X5M Space Grey/Full Silverstone
2003 4.6is Silver/Black