Another interesting fact.
Take a look at the effect that added weight and wind resistance take on gasoline and diesel engines when it comes to acceleration:

2011 335i: 4.9s 0-60mph. Curb weight: 3814 lbs
2011 X5 35i: 6.5s 0-60 mph. Curb weight: 4960 lbs (+1,146 lbs, or +23%)
That's 25% reduction in acceleration at 23% increase in curb weight. A straight line in a chart.

2011 335d: 5.7s 0-60mph. Curb weight: 3825 lbs.
2011 X5 35d: 6.9s 0-60mph. Curb weight: 5192 (+1,367 lbs, or 26 %)
That's only 17% reduction in acceleration at 26% increase in curb weight.

Source: Car&Driver, Edmunds.

Shall we see projected acceleration given additional 6000 towed lbs?