OK my friends my dad has an 04 3500 4x4 dually that he bought off a lot for a pretty good price butt its not a stock truck i think it has a 4.10 rear idk if thats stock or not. it has a cold air intake from k&n and i think it has ben programed into it because it isnt shifting normal. his old truck was exactily the same an it was gettin 26 to 28 mpg on the high way and this one is only gettin like 16 or maby if ur lucky 18 and it shifts funny through the last 3 gears 2nd it gets to like 2600 rpms and then 3rd gets up to like 2600 or 2700 rpms then shifts and drops straight through fourth will the rpms are droping he was thinking that some one has tuned the truck and he was wondering what knid of chip he could get to set it up how he wants it to drive please let me know thanks i dont know anything about these 24valve trucks and there computers its all to confusing thats why i drvie a first gen woohoo

 

Check the TV cable agjustment and see if it is set to tight.

If thats good, its probably the gov solenoid or transducer causing the hang shifts, thats a typical symptom of a failure in that area.

 

4.10 is certainly stock. They only offered 3.73 and 4.10 on these trucks. Look for a sticker in the glove box or on the back of it. It will have the axle ratio for you.

Hopefully fixing the trans will help your mileage a little. I wouldn't expect it to get much better though. Consider yourself lucky though: I get 14 to 16 at best out of mine. . . .