fuel mileage stinks 06 dually

I Have an 06 GMC Dually that is getting garbage for fuel millage when trailer is loaded its getting at the worst was 9 mpg best 10.2 MPG.
Unloaded was 10.9 11.3 and no trailer no better than 12 this is all ranging for 65 mph to 80. No black smoke comming from truck. I just dont get it. The local dealer I bought the truck from does not have a deisel tech working for them, and chevy dealer wont touch it. Can any one give me any insite to this problem and to where you might suggest i take the truck in the are i live near limrick pa and dont say Fread Beans.......

When is the last time it had an airfilter? That was the problem with my truck.

Goodluck.

I've used Bergey's in the past. Always had good luck there.

Good idea about the air filter.

How long have you had the truck? In other words, has this lower mileage just started? Are you trying to drive for mileage, or are you on the throttle a lot (80mph?). How many miles are you figuring into your calculation for MPG? I mean, are you figuring your mileage after only 50 or 100 miles? What kind of weight are you pulling?

I was calculating between fill ups. i had done a number of things to see where i was at with MPG. steady speeds flat ground cruse control on or off all sorts of things my truck has less than 20,000 miles on it i have owned 3 other trucks just like this one never had a problem with MPG before. i always keept my trucks around or under 2000 rpm wich put me near 65 70 MPH. most of the time im trying to get the best economy out of the truck i need to Fuel isnt cheap.

Your loaded averages sound about right, compared to what I see with two '06s. You unloaded averages are low.

Try three tank average with cruise RPMs never exceeding 1700rpms. Check tire pressures on truck and trailer, change air filter, run a good fuel additive double dose first tank, regular mix second and third. When at cruise, turn off tow/haul so you'll get upshift into 6th, TC will be locked at that speed/rpm anyway.

Probably have a few injectors (sprays) not spraying...

The fuel economy on my 01 has always been around 18 to 20. power has always been excellent (even with the injector and head gasket troubles). I've always used Sea Foam, When I change fuel filter I pour a can in the new filter, pump it up and start it up. I also pour a can in the fuel tank. and another about once a month or about every 5th fill up. I Love this truck. I do run a volant air filter that I wash out with the water hose every time I change the fuel filter. No I don't oil it and I think it is actually a K & N filter but it works for me.

You never said what you primarily use this truck for. If it is a construction vehicle or is allowed to set and idle, then of course your mileage will be low. Has anyone checked/ upgraded the software on the vehicle? Are the tires oversize, is the vehicle lifted?

This truck is a daily driver as well as long hauls for transporting cars or cargo. I wouldent try to figure out MPG on a tnk when the truck was ideling for a long time im used to transporting so i have an idea as how to figure out my MPG. This is a straight up problem as to having bad fuel milage like i said. its not complicated ordeal i must have run thru 7 tank fulls of fuel i had plenty on time to figure out several diffrent ways of trying to run the truck. the tires are 235's i hade an 04 ran same set up went to the same places and it was 10 loaded but unloaded was 14 16 no trailer 17 to 20 and this truck has an extra gear so id think i would have better MPG 10 MPG all the time Really stinks. I would think on the highway that the bigger tires would increase my MPG i could understand in a city enviroment that it would decrease it because as needing more tourq to get them moving. any more ideas guys? thank so much for what i have till now.....

Kevin,

Don't get put off by all of our questions. There's been more than a few times where someone complains about low mileage only to find out they were calculating their mileage after taking quick trips to the grocery store (I'm exaggerating a bit, but the point is that in order to help you and get an accurate answer, and unless you provide all that info upfront, we have to ask these seemingly mundane questions). It's all a process! :)

There's been some good thoughts so far, just thought of something else. By any chance are any of your brakes dragging? It's probably a long shot, but it's also easy enough to check. I'm not as knowledgeable as many, but aside from simple things like air/fuel filter or something like the brakes dragging or an actual physical problem in the drivetrain, I'd lean towards the afore mentioned injectors.

well im not put off by the questions. They all make sense. and i appricate you all taking the time to help me. im frusturated because the dealer i usualy deal with now dosent have a good mechanic and im sure im gona betold thier is no problem when thier obvoisly is. and if it cant be repaird i will no longer be able to use this truck.