Left for Phoenix New Years eve morning speed limit +5, tail gate down and outside duals off. Denver to Alb. 16.6, same as last trip with all tires on the ground, then I noticed the Edge was at level 3. Set the chip to 1 and got 18.6 on the leg from Alb to Phx, for an over all of 17.5. Trip home at speed limit, gate up, new bike in back and 6 tires on the ground got 17.2.
From this looks like the tires might be worth around 1.5 mpg and the gate up costs about .5. The 1.5 would save about 5 gal or 10 bucks on a trip to Phx. Doesn't hardly make it worth while to take the wheels off. And besides that the truck looks silly with them off.

Not sure where I saw it but read a report once that someone tested the theory of truck tail gate up or down for better fuel economy and the conclusion was that gate up created less wind restistance than with gate down. Suprisingly adding a bed cover didn't affect the results. Sorry I didn't book mark the test report.

I would expect the milage from Denver (5,000 ft?) to Phoenix (1,000 ft) to be better than the reverse route from Phoenix to Denver. This is because there is more downhill than up hill when going to Phoenix, and more up than down when going back to Denver.

I drive quite a bit between Phoenix (at about 1,000 ft elevation) and Springerville, Az, (at about 7,000 ft). The uphill run uses about 2 more miles per gallon than the return downhill run. (This is with an emty load, milage hand calculated.)

If I understand your numbers, the fuel savings with the tires removed may be less than 1.5 mpg after factoring in the elevation changes.

http://dieselplace.com/forum/showthread.php?t=20588&highlight=tailgate

My first trip to Phoenix using I70 to Crescent Junction then south via Moab, Tuba City and Flagstaff and back the same way got the same milage both ways, 16.6. I suspect the altitude change does make some difference but most of it happens in a pretty short distance coming off the Rim around Payson, the rest of it being pretty gradual. Like I said in an earlier post, the only way I know to get really good milage is always drive downhill.

or, have a 45MPH tailwind. That was dreamy mileage, wish I could get that every day.

My MPG has been great so far this winter. I had a load that went from Cin., OH to Niagra Falls, NY during the snow the week before Christmas. My trailer weighs 6500# and I was hauling 3200# plate. I got just over 15 MPG loaded and over 16 empty. I really have no explination for this other than the six speed.