reach up and pull it out of the headliner, and throw it away. It is called a LIE-O-METER.
Unfortunatley, the readout display is one set of codes that has not been completely dissected and a work around written for. You can have your PCM flashed for 140 extra HP, but no one has figured out how to make the mileage correct.

 

I use my early 99 F350, 4wd Auto, crewcab to tow a Polaris 750, and I get 14.6 mpg.

 

12.5 in town moderate driving 37's on 3.73's, 18 on the highway but i mostly just cruise around 65. My overhead is usually pretty close, within a mile per gallon of actual. I am curious as to why that is not so on all trucks. Yes I check my mileage against gallons used every tank.

 

i range from 12.5 to 19mpg depending on what im doing, when towing the trailer (32 foot toy box) fully loaded with water, fuel, quads, and everything else needed at the sand dunes it drops to 12.5 or so, when driving in town im getting 17-19. unfortunately i cant keep up the pace with some of you guys, at 65mph im at 2100 rpm... 1999 F-350 supercab DRW long bed, ZF-6 trans and 4.88 gears.