I normally get 18.2 MPG according to my overhead computer. But during the last 2 weeks my mileage has dropped to 16!!! That may not seem substantial until I say that that average is calculated using 9000 miles worth of average driving. For it to drop so much so quickly something has gotta be wrong. Other than the 100 miles less per tank of fuel I don't notice any problems. No smoke, No power loss, No dripping at the exhaust, Nothing to indicate a problem other than the horrible mileage. I just changed the oil, air filter and rotated the tires but haven't made any effect. Could an injector be bad or stuck open? Could the clutch or transmission have a problem? Should I just change all the fluids in the drivetrain - tranny, and rear end. Please help! With the price of Diesel at $2.50 these $80.00 fill ups are going to kill me!

 

Are you hand calculating? Are you saying you haven't reset the overhead in 9000 miles. If so, there is only so much data it will average the mileage readout over. Give us truck and driving specifics too.

 

Usually a sudden drop in mileage is an indication that either your IAT sensor or MAP sensor has become grimed up and needs to be cleaned. Plenty of threads on this subject. Hope this helps some.
Mishkaya