I've had my x5 loaded to max capacity and it still has no problem towing nearly a ton of trailer. It's helps a lot that I have the air suspension as there is no sag with 200-300# of tongue weight which pre-loads the spring rate on the back and makes it more stable.

My mpg dropped from 22 to 17 when towing.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)