The DII is heavier Spence, instead of all the body panels being aluminum like on a DI the DII uses steel doors and aluminum everything else, so it weighs in almost 500lbs more than a stock DI.
The larger engine is supposed to not work as hard and get better MPG.
I average high 14's to low 15's as a daily driver with my DI and thats with a SD rack and ARB winch bumper on the front and my fat *** in the drivers seat.
On a road trip I can get 18's.

 

Yeah, I didn't even check the weight. I had gotten 18 on road trips even 21 once on my d1, but if my truck can't even get 16mpg doing 65 instantaneous mpg, then 17 or 18 is impossible

 

Don't know what's wrong with my truck, but my stock 2000 gets about 15-16 around town and about 20 on the highway. I drove from Reno to Sacramento a couple weeks ago and got 21.4 from gas station in Reno to gas station in Sacramento...

 

The leaking air bags on your P38 provide some extra thrust

 

Shhhh, don't spread this around or the oil companies will get ya.