the ones on eBay DOES INCLUDE the wheel arches. TPMS on 18" wheels should work fine with 21".

"You will get four OEM wheels BRAND NEW in original BMW packaging. This set includes the required wheel arch extensions you need with these wheels."

BMW X5 V Spoke Style 239 European 21" Wheel Set of 4 :eBay Motors (item 190218452037 end time Jun-22-09 14:49:05 PDT)

__________________
2011 BMW 740i
2011 Toyota Sienna Limited
2008 X5 3.0 Space Gray (sold)