It's more a matter of wind drag than the weight. I don't get much worse mileage towing a 7700 lb. Kubota tractor on a dual axle trailer than I do an enclosed trailer with two 700 lb. ATVs in it and got significantly better mileage when I had my 2300 lb bass boat and was fishing tournaments regularly.

 

I get 10 - 11 MPG with a 2009 2WD CC 3.21 gears towing using tow/haul mode. Towing speeds around 65, more on the downhills. My trailer is 8 ft wide and almost 10 ft tall and 19 ft long. It's a light weigh so loaded it weights about 3500 lbs. I carried firewood, extra water etc in the bed for about 250 lbs and then the wife and I plus tools and other items the cab for about 400 lbs.

Biggest difference for MPG were Hills, head winds and tailwinds. In the flat with tailwinds and OD I was getting over 12 MPG. Hils and head winds I was getting between 9-10 MPG. These numbers are hand calculated.

You can see my set up in my members profile

 

between 9-13 mpg towing my 5000 lb travel trailer. Thats with the 3.55 gears and keepin it at or under 65 mph, and tryin to keep rpm's under 3500..

 

I was getting close to 16 this past weekend towing a friends car to the dragstrip. 2800 lb car + open trailer + gear. Somewhere around 4,000 lbs. This was at 65mph. Hemi CC 4x4 with 3.92 gears. Truck was still going into eco mode some of the time even with the trailer. Never took it out of overdrive at all, trans wasnt shifting up and down, so I left it. Are you guys using tow/haul mode? I dont think its necessary unless the transmission cant stay in one gear. My trans temps were only 170 degrees.

 

I get about 14 mpg towing the cars around on the trailer - about 5000 lbs with 3.92 gear.

 

I did not go into eco mode unless down hill. I think the big diff here is campber vs car trailer. the campber has a larger foot print on wind , the car flows behind the tucks wind tunnel.
My trans temp was usually 175-195 at highway speeds.

 

I'm getting about 10-12mpg (readout on truck) pulling a 7500 lb camper. I have it in tow/haul mode and drop it to 4th gear. I keep it around 90-100km/h (55-60mph). It revs at about 2500rpm. I tried it in 5th, dropped to 8-9 and was shifting up and down alot, but I was in some fairly big hills. I am installing some air bags in the back to help lift the weight, maybe it will help with the mileage too....we will see.

 

First time towing my new 29ft Jay Flight travel trailer. Weighs 5500 Dry, total weight about 6000lbs. About 9 MPG keeping it around 60/65.

 

I just did a 20 hour round trip with my truck and trailer. Loaded, I was weighing in around 7500-8000 in box and trailer. I went across BC and up and over the Rockies and over a couple different mountain passes. I averaged (in American) 8.5 mpg. I was hoping for 10 MPG (canadian) and got 10.2 so I am happy. Not extatic, but happy. I found that running at 95 kph versus 105 kph made over a 1 mpg difference. I also found that on one leg I filled with 90 octane (the lowest they had) versus the 87 I was normally running and got better mileage. Now, the legs were all very diferent but this was a mountain pass leg up to Rogers Pass (elevation 4,534 feet/1382 m) from Golden (elevation 2,625 feet). So a good hike up. I also had it out of tow/haul, but latter tanks at 87 out of tow/haul noticed no increase, so I guess it must have ben the Octane boost. Next trip out I will try the higher octane and see what happens.

I did notice tranny temps WAY higher then I would have liked and will need to look into that.

 

TI 3VOM, try keeping it in tow/haul mode, that will keep the tranny a bit cooler. I run 91 octane every once in a while and it makes a pretty big difference, don't know why I dont run it all the time!