Overhead display shows 99.9 miles per gallon every time I start the truck. While driving it settles in around 15 to 20 Mpg. When I accelerate fuel mileage goes up to as much as 22 Mpg, when I am going downhill at 0% throttle mileage decreases to 4-6 MPG.. again increase mileage under hard acceleration decreased mileage when decelerating or going downhill.

Never been good display from the get go. 

 

I've got to ask do you have oversized tires (bigger than 265/75 R16)?

 

Do you have any kind of tuner on the truck? (Smarty, Quadzilla, Edge, etc)?

 

Negative, tires are stock, no tuner.