With so many variables, you are going to get what you get, mpg-wise. With the Lance Lite on the bed I was happy on many trips getting up to 15 mpg/hand calc. when I had stock tires and wheels, no winch; no upgrade drivetrain parts; no beer, no capability. On 3 trips in the truck camper this spring into summer, the splay was between 11.7 to 14.8 mpg. One variable on the last trip was having two bicycles on the front hitch. At 65 mph, this ate into the mpg much more than I would have thought. I could feel the wind resistance. Another variable on the same trip was the slower speed that we traveled: 40 to 50 mph with some big hills. The mpg shot up to near 15 even with the bicycle wind break on the front and the hills. Another variable was our NV5600, six speed manual trannie. 365 pounds of beef. Cast iron case. No intermediate bearing needed. It's not going to deflect. However: obsolete. Too heavy. There is no speed at which you move that you cannot get into some sort of sweet spot with the rpms between 1600 to 1950 rpms for maximum efficiency. The old Gen II High Output Cummins has torque (505 # ft) all the way down to 700 rpm, unlike some of the newer diesels that sport almost twice the HP and TQ, but in a narrower band. It feels pretty puny now, by comparison; but with no pee canister or DEF tank to spoil your holiday.
The real question is not what mpg you get, but what you do or fix to get the maximum result out of your set up. There was a study done by some governmental entity about 4 or 5 decades ago (so now it is obsolete) that tested a lot of cars to see what the best SPEED was to get the best mpg. In other words, what speed wind resistance and tire deflection negatively effected efficiency. They came up with 43 mph. I know this probably does not apply to trucks with campers on the back, but it was interesting that chrome boat cars from the 60's with bias ply tires, no wind tunnel testing, or smooth slip stream found that speed the most efficient. This corresponds to our mpg while traversing the Mojave Road: 150 miles of dirt and sand. We drove for 4 days, with tires aired down to 30 pounds air pressure, @ about an average of 10 mph, and the fuel gage finally got below 3/4 of a tank.
So speed has a LOT to do with mpg. Gas or diesel has a lot to do with mpg and speed. A diesel uses very little fuel at idle compare to gas.
So, not so simple. Just run whatcha brung and have a good time.
jefe
