Ok, I'm sure this has been asked before somewhere in this forum, but I'm thinking of going 20 inchers for my X3. What kind of offset and tire sizes should I be looking at? I know the stock is 46, however, I'm looking into some wheels which have a 40 offset and also I'm looking into staggering front 8.5 and rear 10. Does anyone know the optimal tire sizes if I go for this setup?

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2004 BMW X3 3.0i in Silver Grey
2003 BMW Z4 3.0i in Sterling Grey
2003 Mercedes E320 in Brilliant Silver