I don't know what the optional equipment is, but it shows up as weighing 233 lbs, and doesn't cost anything in the configurator, over and above the hitch price. I think it is just a way of flagging the diesel model for the rating.

Yes, the Euro hitch is a different design. It isn't available in North America as it doesn't meet the safety regulations; that doesn't mean that it isn't as good, just that it is built to a different standard.

The E53 had a different tow limit in Europe as well, well over the 6000 lbs we had in North America. I thought it was around 7000 lbs, from memory.

The X5 35d has been reported to have a 6500 lb tow rating in North America, as a result of the heavier automatic transmission that is also used in the V8 model. Not sure about the accuracy that, but it was listed in an Autonet review. I would want to see an official BMW rating, but the logic is reasonable.

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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