I believe DownTheAvenue's point is that this type of information is the owner's manual. After 16 hours, still no answer while it may take 3 minutes to look up. I often find the
electronic copy more useful as I can search keywords.
The DIC information and controls are listed on page 5-29. It explains the fuel economy is an average since last time the average was reset for example. It explains how to reset it. It looks like there is an instantaneous speed and an average speed. Maybe he is looking at averages? Does he reset all the data every time he fills the tank?
It is possible the truck does not have the correct data for gearing/trans/tire size or even fuel tank size which affect the calculations shown on the calculations. Only a dealer can confirm and configure that.
Bob, can you explain in more detail or give examples of what you think is off?