try the hss stiction eliminator in the oil and adjust the valves... mine is really consistent tank to tank under normal commute miles, averaging right at 21.0x mog over the last 30-35k miles. fixed a tune problem and now the last 2 tanks are at 22.4x mpg. it was a revert to a known good tune to get the change which led to an ah ha moment which still needs to he pushed into the tune. once that is done i will bet there will be another jump in mpg.hello, so I have a 04.5, 233k miles. 2 months ago my mileage dropped from 23-24 to 16-17 highway. ( hand calculated) original owner......replaced injectors because it was time, no change. Sent turbo(35k miles on it) back to shop for checks,, everything fine. Replaced MAP and FCA,,,, no changes.....fuel filters, air and oil changes done. Ot seems no one can fix when this goes wrong. Some people don't believe trucks can get good fuel mileage. However I have 233k miles and 16 years with a truck that did, no it has stopped. So I feel your pain.
fwiw, there is a ton of info here that i have posted that can pave the way for anyone looking to improve their mpg. an example would be to use a 2nd air dam under the swaybar mount on stock height 4x4s. if you love to speed & do 75-80 often, its good for about 1 mpg from what i can tell. remember, 19.3 thru idaho & into utah doing 85 for a couple hundred miles on 2 trips, one with air dam got the 19.3 & 2nd trip did mid 18's without it.