Some really good info here from withidl.

I agree that the X5 is an excellent tow vehicle (I don't tow anything that heavy, but I have been impressed within the limits of my 3.0/5000 lb tow rating. The very short distance from the axle to the hitch centreline is a really big factor in the stability.

I think the published BMW limit has more to do with the preparation carried out by the average owner towing, and BMW needing to make sure there are large safety factors. I don't think many owners, if any, take as much care in setting up for towing as withidl does, from the sway bars to the trailer loading regime. The manufacturer needs to address the liability risks created by the lowest common denominator driver.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue