I have a 2007 5.9. Last year I was getting around 17 MPG. it slowly has gotten worse. now I am averaging about 12 to 13. Truck runs like it did new still. Doesn't matter if I am towing 7000 or just driving. 12 to 13

 

Seems like a dumb question but are you keeping up with your oil and filter changes? Air filter? Codes??? Fuel filter?

 

Where are you getting the mpg numbers? Hand calculating? Do you have a loose hose on your fuel tank? Pin hole in the tank? Leaking fuel somewhere?

Maybe the neighbor has a diesel and he siphons your fuel off at night?

 

Yeah I am up on all my filters. I actually just put on a stage 2 AFE intake kit and a Afe bladerunner manifold to see if that would help. Still nothing. I have checked for any leaks and there are none. I am at a loss for words

 

Have you checked for dragging brakes? Are you doing more short trips now? Where are you buying fuel? Do you have any white smoke or rough idle first start of the day?

 

I usually get my fuel at sheetz or citgo. But now that you mention it some mornings it seems like it's not idling real smooth.

 

Have you taken it to the dealership for any reason. If they flashed the computer ( which they will do when you take it in for any reason ) and put a update in it your milage will suffer. I had the dealership replace a fuel lift pump under warrenty ( after failure ) and they flashed my computer and told me it was new software,update. My mileage dropped almost 4 MPG . Had to go to Cummins and have them reset the original parameters to get my mileage back , cost me 250.00 but it was worth it

 

If you have white smoke and a rough idle on the first start of the day, along with reduced fuel economy, you may have an injector going south on you. My truck had those symptoms and new injectors cleared them up.