I was watching my ECU's output, specifically looking at fuel rail pressure, fuel flow rate and LPHK. It seems to be directly in tune with the loud pedal position, vehicle speed and engine load.
My guess is that the ECU knows quite well how much fuel is squeezed through those injectors at a given pressure for a given period of time, so once it knows how fast you're moving it's just simple maths to arrive at the LPHK figure. Deviating from standard tyre sizes will provide incorrect inputs to the ECU which of course then provides incorrect outputs, of which I am guilty.
I was in ARB yesterday afternoon booking my car in for a Long Ranger auxiliary tank and the guy suggested that the auxiliary tank empties into the main tank first, then the main tank starts to empty and the fuel gauge begins to display as it normally would. I guess I'll get used to that.