In case you decide that RPM's may be effecting mileage.
Here is a neat little calculator you can play with. You can input your info and it will figure rpm's for you. http://www.csgnetwork.com/multirpmcalc.html
I believe the stock tire size is 31.6", 285/75/17 = 33.8",285/70/17=32.7" or you can go to the tire manufacturers web site and get the size for the specific tire. Use 1.00 for transmission final gear ratio for DRIVE and .69 for OD.

 

I get around 15 mpg towing 6500 lbs, but I run 60-63. Trying to run 70 in the mountains in headwinds is going to use some fuel.

 

Do you pull that in OD?

 

Fill in your sig,,, with how your truck is equipped,,, I get 15 pulling similar weights @ 60-65 mph,,, like you say,,, if I'm running 70+ it drops to 12-13 or less,,,,

 

While towing my 5th wheel which is 13,200 loaded I get 10 over 1900 rpm's.
If I can do 55 I can get 15, to bad 55 is so slow.

 

I get 14MPG towing my 30ft 5th wheel. I generally tow at 65 in OverDrive with no issues. I can slow down to 60 in Over drive with no issues. No we do not have the mountains in Michigan like out west but i pull through PA on the turnpike and West Virginia (Blue Ridge mtns) with no problems in OD. I have 3.73 gears too with stock 265 tires. I can not imagine your taller tires causing that much of an issue. I would look to some type of mechanical issue. On any of the hills I have pulled in the Blue Ridge Mountians with my 5'er if I layed into it she would gain speed. The truck is bone stock. Guess I got one of the good ones.

 

That is really good,,,,,, what does the 5ver weigh?????

 

Three differnt trucks same 28 foot TT weighing about 8500 - 9000. '99 Powerstroke - 9 mpg, '04 Powerstroke 9 - 10 mpgs. '06 Cummins in sig 9 - 13 depending on speed. The best mileage I ever got pulling this camper was up through the local mountains to TN. The speed limit was 55 and there was enough traffic that I had to obey it. I got 13 for the trip. I have not found a hill yet that would cause me to downshift. The difference is my truck with the manual trans is geared like an auto with 4:10 gears. 2000 rpm = 63 MPH.

 

Stock 2wd, 03 with 225,000 miles. 235/85/R17 aired to 70 psi. 3.73 gears.

Scaled in at 27,800 with a 4 car carrier. 10-12mpg @ 65mph.

 

Pretty good!!!! is it a bolt action,,,, or an Autoloader?????

 

I pull a 33', 11,000 lb fifth wheel around the inner mountain west (Ca, Nv,Utah, Az, Or) . Lots of mountains! I now have 295/70 Toyo M/T's which I believe are just under 34" dia., and 373's. I drive at 60 to 65 mph, mostly 65. I cosistantly get 9 to 10.5 mpg depending on the number of hills and wind. I only towed it about 750 miles with the stock tires and they got me about 11.5 to 12 mpg's.

But I only kick it down to 3rd gear on steep hills, never on the flats and it pulls with plenty of power. I don't think I have ever had to put the accelerator on the floor for any hill climb when towing.

Earlier this year I was towing into a 40 to 45 mph head wind for about 100 miles on the flats and had to run in 3rd to maintain 60 mph. That dropped the mpg's down to about 6 or 7.

 

Towing my 20' car hauler with the Bronco on it, about 9K, it gets 12 mpg on relatively flat terrain in OD with the cruise set at 70 mph. There is probably more wind drag on the camper, so that may affect your numbers. I'd like to do a trip at 60 mph to test the mileage, but with highway speed limits of 75 mph I just can't do it...

 

13 high MPG in 5th gear (1800-1900 RPM) @ 60-65MPH
12 high MPG n 4th gear (2200-2300 RPM) @ 55-60MPH
I'm use 5th in the flats and 4th when I hit any kind of hills.
Over 65 in the flats it starts going down fast due to wind resistance.
#'s hand calculated on full tank refills.

 

6spd manual. Engine is the HO.

 

I average between 10.5 and 11 towing an 11,000lb fifth wheel. I never tow the fifth wheel more than 65. Thats the sweet spot at about 2100 rpm. Pulls really well there and fuel mileage is acceptable.

I average betwen 14-15 towing a 5500lb boat. I don't pull more than 65 with the boat either. Its got ST tires that aren't speed rated at more than 65.