The trouble with chains on 20" rims would probably be the chains gouging into the rims.($$$) I think that's why BMW recommends 17" rims if using chains. You really want a narrower tire for snow.

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Mark _______________________________
2001 X5 4.4i topaz blue/sand Sport, complete Dinan S2 , hitch, V1, tint, BSW Stage 1, Alpine INA-W910BT
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