Thanks for the comments. My experience was that I had no effective retarding force with my Passat 1.9 TDI and manual transmission, or my Landrover Discovery TD5, when compared to their gasoline-powered equivalents.

I agree that a 5000 lb trailer with functioning brakes is not an issue behind an X5, diesel or gasoline. It is just that as I live in the mountains, and often see Dodge, Ford, or GM diesel pickups pulling large loads equipped with some form of aftermarket engine brake to avoid brake overheating, that I would hesitate to call the X5d the ultimate tow vehicle. At reasonable loads I am sure it will be great.

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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