If I get 20k miles out of the rears (20in w/325mm width tires) I'm happy. As your supplier mentioned, X5's are known to go through rear tires.

You didn't mention the brand of tire or size, but if looking to maximize life, you might need adjustable rear arms to zero out the rear camber and have toe in limited.

I'm sure others with more knowledge of alignment specs relating to tire wear will chime in.

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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
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)