IMO, the instant fuel mileage reading is to be used as a guideline only to assist the driver to maximize the amount of fuel being burned at any given moment.
The digital readout in our trucks reminds me of the old vacuum "Economy Gauge" that car manufacturers put in some cars back in the 1980's.
If you had one of those gauges, you learned to keep your foot out of the gas in an attempt to keep the gauge's needle in the "good" area.
If you had a light foot, the engine's vacuum stayed high and the gauge gave you a nice reading.
Of course, there were some that mashed the gas at every opportunity to see how long they could keep the gauge in the red.
One of my past cars was a 1980 Pontiac Turbo Trans AM.
On that car, Pontiac saw fit to install a "Turbo Charge" gauge on the hood scoop facing the driver.
As the turbo's boost increased, another one of the three indicator lights were illuminate, until all three were on at maximum boost.
I can't image that gimmick did much to increase the fuel economy of that series of cars.
Chevrolet Z-28's of the same time frame had an electrically operated set of air inlet doors that would open when you smashed the gas to the floor.
And let's not forget the Chevrolet Chevelle SS series and their pop-up air intake on the hood.
Dang, that was a nice walk down memory lane.
Now, what were we talking about?