Heavy Duty Payload Package. AKA max payload capacity

5.0 HDPP requires regular tow package.

 

IMO the 5 litre no MTTP might have something to do with low end torque but probably as more to do with Ford wanting to make the 3.5 the top dog motor.

I really think that Ford doesn't want to sell too many HDPP trucks. They exit for bragging rights about best in class. They hurt CAFE numbers and they can't be cost efficient to build, the additional cost on mine was less then a radio upgrade!
The dealers don't help the situation, in all of Ontario Canada there are 2 dealers who stock a few HDPP trucks most have no idea of what they are. This is really too bad because the HDPPs are very capable vehicles. My daughter has a regular F150 with almost identical specifications to mine so I have a good bases for comparison, and they feel like different trucks, As for MPGs I'm actually doing better then her due to to driving habits (I drive like an old man!)

 

I believe, for sure, that the issue is the low end torque. At 2000 RPM, the 3.5L EB is putting out almost 100% torque, and is in the neighborhood of 200 ft-lbs higher than the 5.0L at the same RPM. What it means is that going up a significant grade, the 3.5L EB will be tooling along at 2000-2500 RPM, and the 5.0L will be in the neighborhood of 3000-4000 RPM to keep up. The 3.5L EB will just be a more relaxing tow in demanding situations.