I have a similar truck, except a 4 door with the 275/70/17 tires.

The overhead readout is consistently 1 to 1.5 mpg optimistic. I hand calculate each fill up. It averages 10 mpg locally and 14.5-15.6, depending on the road (60-70 mph)

The odometer is pretty close, so the problem is in the algorithm they are programmed with.