It's all areo dymamics. With the various setups you need to find the best speed to keep the airfolw going over the truck and trailer in one smooth motion. It is possible to tow a heavier trailer and get better mileage if you get that airflow going correctly. Yo can tow a litter trailer with a frontal spuare footage of a barn door that weighs 2klbs and get the worst mileage yo have ever seen.
Just keep triing and find that point where you get the best mileage for the highest speed

 

On my first towing trip, pulling a 6.5x14 flatbed with two ATV's, and about 400 lbs of stuff in the bed, I averaged just under 12mpg over 500 miles, running 75-80 mph. I was kind of disappointed as well, even when considering that the V-10 SuperDuty I traded in would give me 7-9 mpg at best under the same conditions (maybe 10 mpg unloaded), and would struggle to stay in OD when encountering even a slight headwind or grade. (3.73's, 315's)

After thinking about it, though, the Dodge only had about 600 miles on it at the start of that trip, in the 4000 miles I've put on it since then, I've noticed considerably better response and my mileage has gone up as well. I'ts too hot to make another Dune trip anytime soon, but I'm sure that our next trip will net us considerably better mileage. It's been mentioned many times on this and other forums that the Cummins takes a little longer to break in and loosen up than most engines, combine that with the stories we all hear about the guy that consistently gets 20mpg on the highway, and 12mpg sounds like a real disappointment, while it's probably about right for a tight new truck under these conditions.

Anothe factor to consider is speed, obviously the faster you go the more fuel you'll burn, but there seems to be a real drop in economy when going from 65 to 75 mph, my truck, unloaded with cruise set at 65mph will give me a consistent 18.5 mpg, taking the same(30 mile long) stretch of road at 75 mph will net me about 16.5 mpg. Do I expect it to get better? Absolutely, I'd like to see that 16.5 mpg at 75 while towing, but meanwhile I take consolation in the fact that I'm already getting almost 50% better fuel economy than I did with the SuperDuty, with a ton more torque as well.

 

Look at the torque/hp rating from the late 80's on the CTD and where were at now. Torque goes up.....mpg goes down. Maybe when were at the same torque/hp ratings as the big rigs with the Cummins we can all enjoy the 5-6mpg they get.

 

I pulled a 3k lb boat from Great Falls to Kalispell a week ago (All up hill, no joke) and got 16.5 mpg. I was kind of disappointed, but I guess I am doing pretty good. Had fun blowing the doors off a Power Joke on a pretty steep grade (He was pulling a couple of jet-skis). I was grinning ear to ear and I think converted a GM lovin buddy that was ridding shotgun. I think he enjoyed the pull more than I did! -Steve

 

I pull a 24 foot artic fox, loaded with water, 2 motorcycles in the truck with a 5000 watt generator and 4-5 5 gallon cans of water, trailer loaded. Total vehicle wt at the scales was over 15,000 pounds and I never got below 14 mpg going down the highway. This was in hilly country so although I had to go slower than 70-75 I also had to pull some steep grades. I am happy with this number. I think the manual would do a little better.

 

My first trip towing my trailer (20' enclosed V nose about 5000# with my quad & gear) was at 1100 miles on the clock, I got 11.8 MPG & averaged 70 MPH on a 350 mile round trip.
On my last trip with the same trailer, load, route & speed I got 14 MPG, I have 16,000 Miles on it now.

 

I have about 8000 miles on mine. I get 9-10 mpg pullinm my 7000# TT at 65-70 MPH. I can get 16 on the interstate if I keep it at 70 MPH or below, but any amount of city driving drops me right back to the 12-14 MPG I normally get.

This is my first diesel and when I bought the truck, I was hoping to get better fuel mileage. I don't really get any worse mileage that with my old truck (2002 Chevy Avalanche 1500) and I figure I still do better that with a 3/4 ton gasser. Figure in the cheaper diesel price that gas and I guess I'm still comming out ahead. Still wish the $5,000 diesel option would do a little better though.

 

I just got back from a 450 mile trip towing 6k of lumber in the mountains and averaged 15.5 mpg with speeds anywhere from 55 to 75. I did have the box turned off. The 03 HO is no doubt easier on the fuel!

 

I agree 9-10 may be a little low, however you do have the 4.10 Towing setup and that may be about right. The other question is is that hand calculated mpg or off the overhead
I tow my 10k 5VR and average 11-13mpg, but I only have the 3.73 rearend
I would not complain in your situation, you could be getting 6-8 with a gasser pulling the same TT

 

You are right SuperGewl, it could be worse. Especially when you figure diesel has been consistantly $1.69 here while gas has been $1.85-$2.05. Anyway, still nice to complain a little every once in a while!!

Also, I have been hand calculating all my mileage. Overhead seems to be off a little.

 

My fiances dad has the new cummins 600 and pulls a 36' 5er. He has the 6 speed and 3.73's. Nothing don too the truck. and it is a single wheel 1 ton. He pulls at about 75-80 mph and gets 14-15 mpg. if he is fighting a head wind he still gets 13. All of this is converted to US mpg. And the 5er UNLOADED is 15,900# With it loaded it is up around 17,000#. At the scales with his generator in the box and the trailer GCWR is 22,986#. Must be the Canadian #2....LOL

 

Ok, I'm sure there are others thinking this......There is no way possible that truck is getting 14-15 mpg towing a trailer, matter of fact it MIGHT be getting that mpg empty running that speed.

 

He is getting 18-19 on the hwy empty. I am just going on what he is telling me and what i have seen on the OHD.

 

Sounds a little wonky to me. If I got that kind of milage towing that big of a trailer I would never unhook it because thats as good or better than I get empty. I only got 12 pulling 2 atvs at 72. I realize that every other country in the world gets better quality fuel than the U.S. but I dought it makes that much of a diference.

 

I reset my overhead with every fill-up, and it consistently reads 1-2 mpg low. This last fill-up my overhead read 17.5, but I calculated it at 19.8. The discrepancy seems greatest at higher speeds. My mileage improved from 18.8 over the first 15k to 19.5 from 15k-22.5k. What I'm saying is, don't trust the overhead, give your truck a chance to break in, and don't expect high mileage over 65 mph, especially pulling a load.