The X5d is the only way to go for towing. 425 ft-lbs of torque from the factory and better gas mileage than my old 335i which had the twin turbo n54 motor. The X5 35i is the single turbo n55 motor which has 300 ft-lbs of torque and has lower fuel economy than the d. I two my miata race car without issues and I get 23 mpg. Trailer and car are estimated at 3500 lbs.