05 3500 Mpg display problems

[Palerider292]

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.

[Mopar1973Man]

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)?

 

[Palerider292]

Negative, tires are stock, no tuner.