Ricohman, what rear end do you have ? Mileage is a lot different depending on what rear end you have?
Mine is 4.30 and while towing 13K I get 10.3......solo I get 16.4 which is good for the max tow pkg. I've only got 4800 miles on it.

 

When I got my '15 F350 DRW, I had about 400 miles on the clock, and hooked to my 41' Raptor loaded to the gills, and headed for AZ, across Montana, Idaho, Utah, Nevada. When I got to my home in AZ, I was showing right around 9 MPG average for the trip. I had set the cruise at 68 and just drove. Up the mountains, down the mountains, through the side winds, whatever Ma Nature threw at me! It was the most relaxing trip ever while pulling my Raptor!
I now have 13000 miles on it with approx. 5000 towing my "Little" trailers (Raptor or 38' enclosed gooseneck) and show an average of 11.1 MPG since day 1! When I came back to ND this spring, I had an 18' flatbed (empty) hooked behind the gooseneck!!! (the gooseneck was loaded close to max) and showed a trip average of 11.1!
Of course, the gooseneck doesn't catch near as much wind as the Raptor so that makes a huge difference!

 

I have 3.55's. I like the deeper gears but this was the deepest I could get.

 

I drove 680 miles yesterday with my work trailer that is 18' long, 92" high, front V nose, and weighs between 9-10K.

I averaged 10.7 mpg running 70 mph and had a 6mph headwind/front quarter wind all day. I carry 86 gal of fuel so I typically run at least 350miles between bathroom/stretch stops, so I only stopped moving 3 times. Basically Greenwood, SC-Baton rouge in just under 10.5hours I think.

I took pictures of my dash but they haven't appeared on my iPad yet so I will try to post when I can.

My temps were generally 221° oil, 198-202° trans. After a few minutes idle it would drop to 197° oil, ambient temps were between 94-100° all day and no rain.

I had 11mpg for the first 220 miles of the trip, two lane hwy and 65mph through/around ATL, so I know I waste fuel going 70. It's hard to drive 680 and sacrifice time trying to eek .5-1 mpg....

Hope this helps. It's not great mileage but speed, heat, wind, tall trailer all costs fuel. I also run in M6. There are times when I know it would downshift both to climb and descend hills, I hate that. I like to let the motor and turbo work it out.

On edit, I rolled 44,000 yesterday. I think it's broken in. Tires freshly rotated, 72psi cold (trailer too), T6 oil, K&N CAI, bed cover, non bio diesel (I filled up at home at $2.39/gal!).

 

The fuel economy is disappointing but it is what it is. But this new F350 gets better economy pulling the FJ40 on the car hauler than the FJ40 does by itself running down the road. Now that is amazing since the truck weighs nearly twice as much as the Land Cruiser.
I'm going to change the oil and give my new truck a quick once over. I have zero issues with how it runs and drives. It does have a rattle in the rear door and the dealer can look at that when it goes in for a radio replacement.

 

I miss my old land cruisers, other than rust they were bullet proof.


with my f350 I notice a drop in fuel economy pulling a light enclosed trailer. I agree with another poster here that wind resistance / drag is the main culprit.

 

My wife's escalade broke down once south of Boston do i went to rescue it with my 6.7 and a car trailer. The trip home from Boston to Delaware i got 14mpg. Not much hills on that drive but it was nice for the ford to rescue a gm product!

 

You DO know that Jesus was a GM man, don't you???? He either walked or rode a donkey everywhere He went!!!!!!

 

My Bible indicated that he preferred Hondas. It says that he and his disciples were all in one Accord - although they were either really crammed in there, or they used to be bigger than the ones built today.

 

OOOOOOOOOOOOOOOOKKKKKKKKKKKKKKKKK!!!!! That works too!!!!! LOLOL