Hi all. I need your opinion here. When I am planning drive my X5 4.8is to off road, I will use 18" rims with 255/55/18 all terrian tires instead of using my 214 style 20" OEM rims. I am thinking of using spacers when 18" rims are fitted on. According to your experience, what size should I use for front and rear respectively? Thanks for help!

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--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.