I've always calculated mileage by hand, based on a full tank to start, how much to refill it again. Been "close enough" over the past 50+ years that I've been driving and worrying about such schit.
But here's the thing: That worked fairly well with trucks that had short fill tubes, but it's still a guess as to how much gas yo put in to refill it, etc. On my old Chevys with the tank behind the seat, no sweat on knowing the tank was full again, for example.
Clicking the nozzle a few more times in the belief that you've now put enough gas in for a good calculation, still leaves one guessing as to the actual amount of fuel used, based on refilling the tank.
The fuel use technology on newer trucks so-equipped, is measuring the actual fuel use to calculate the MPG "as you go".
So if there's a small discrepancy between the fuel use readout and your "hand calculation" method, who's to say one is better than the other?
