one of my customers has an 04 lly and usually gets 10-12 highway with an enclosed car trailer. he/we just added the k&n kit, and 2 modules with an egt. we will se how it does soon.

 

it is all in your gearing in the rear end ... my dad has an 04 lb7 and gets 14 average he has the 4.10 rearend ... i saw another truck same everything and it got 20-23 with 3.73 rearend ... there you go

 

diesels are built to such tight specs that they usually do get crappy milage till there broke in. The best way to break one in is tow a heavy *** trailer 1000 miles or so and put some strain on it. I know a few guys who picked up 2-3mpg after breaking it in good towing.

On diesels also getting a free flowing intake,exhaust, and advancing the timing all lower EGT's and increase "fuel" milage by allowing a more complete burn in the cylinder. My old cummins with 238,000 miles on it and 3 speed tranny got 23mpg on the highway an 20 in town.

 

he installed the egde box and a k&n kit. milage went from 10 to 13.8. he isgoing to montana on monday loaded one way and empty the other (28 foot enclosed trailer).
the track has 110k miles.

we also stacked an edge ad an md-8 on a '04.5 cummins and found a ton of power.