You're making a big error thinking the stock MPG readout is more accurate than any supposed math errors most of us will make dividing out our own mileage.
Mine is consistently several MPG "optimistic".
If you want real world info you should be asking for the opposite: ONLY real, miles driven divided by gallons pumped figures. It's not complicated and your average guy can handle that without error.
The dash readout just isn't the precision instrument you're imagining it to be.
Also, I wholeheartedly agree that buying a diesel pickup isn't about gas mileage. My old 454 gas dually is infinitely cheaper overall to own than my Cummins truck is, despite the better MPG.
But that Cummins can run forever at full throttle without coming apart, and has twice the torque.
That's what it's about.