I am unfamiliar with 18" tire sizes but you could definitely go taller. Wouldn't go much wider on an 8.5" wheel. But taller would definitely clear the fenders. Let me see if I can find the wheel/tire size calculator I usually use...

I just noticed how high the offset is on those. Be sure to check clearance at the strut/spring.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
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2010 X5 35d Build 02/10
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