Tough to compare against Morpheous - he has probably one of the perfect combinations; year, manual, tire size, etc.

One factor that also affects towing mpg is the air temperature. I noticed in the winter when it is -6F outside, the mileage drops even further than just the effect of winter fuel

Like some have said, huge factor is the road you are driving on. Pulling an empty gooseneck (40' - 5000lbs), the truck won a trophy for getting around 15 mpg while in Coloroda / nebraska. The road was flat and straight for an entire tank - I think I drove for 3 hours with the cruise set, never touching the brake, simply chugging right at 2000 rpm. As soon as I entered Iowa, that 15 mpg was gone, baby gone. Averaged around 11 mpg.

What I also noticed is that I was able to get significantly better mileage with the gooseneck trailer than with the enclosed, bumper pull snowmobile trailer due to the wind resistance. Since my truck has a flat bed, the air come up over the cab, back down to the flat bed then WHAM, hits the full front of that snowmobile trailer which is 9.5' high and all of 102" wide. With the gooseneck somewhat tucked in behind the cab, I only have 18" of bug markes on the top of the front bunk of the gooseneck trailer. My next idea is to get a semi fiberglass air dam and install on the headache rack to help direct that air over the 18" of exposed trailer. If I had the ching, I would have ordered the gooseneck with a tampered high bunk or a 6 foot inside height versus the 7' inside height but the 50% off price for a used trailer was way too enticing.

 

i'm towing a 38' 5er weighing in at 14800 and I just towed from Pa. to Vegas on I-40. If I keep her under 1950 rpm (70 mph), I ran an average of 10.8 mpg for the 2400 miles. Running Toyo 265/70 HTs.
Most of the time in OD

 

pulling a 15000lb bumper pull trailer and hauling a 2000lb cab over camper I am currently averaging 12 on the overhead and 11 hand calced at between 60 and 65mph with a 6 speed stick

 

34' TT: 15 mpg at 63 mph on flatland Interstate

 

14k lbs GN loaded with a jeep and a bronco. its actually 13600 loaded. i run a hand shaker mind you with 3.73's and 285/75 R 17's in an all terrain

From the central valley of CA to big moutain MT (whitefish) through the gorge etc or up donner and back to the valley

i always watch RPM's not my speed so much. on a hard grade i pull 2100-2300 rpms otherwise i try to keep things at the 1900-2000 rpm range on hills and on the flats i keep her at 1800. loaded i average 12.2 all around. flats and taking it easy with my 9200 lbs bumper pull trailer i get 13.4. with a 7000lbs trailer lightly loaded and taking my time 14.7 and staying at or below 1900 the whole time. so its not speed asmuchas it is rpm but anything over 70 mph and my mileage drops to about 11 but my 12v did the same thing since my rpms are relative to the speed.

 

Have you checked your trailer brakes? Any drag there?

 

If you go to 4.11 you most likely will never get your money back! So I would be looking at something else.What was your millage in the mountains ? This could pull your millage down fast and it is hard to get it back when running in to a head wind.

 

Mileage was 7.9 hand calculated, running 57MPH with OD off.

 

I made a 6 month trip out west last winter.I put 6.866 miles on the old girl pulling my fifth wheel. For the round trip we got 13.9 miles per gal. Some of thees was jest the truck. I would say 2.000 were jest the truck. Loaded my weight is 22.900+ I try to keep it around 62-63 MPH. I shift it by the boost and the pyro Never let it go over 1350.