But I'm just a fresh PPL, what do I know..
not that much, so you might not trust anything! and probably land immediately if possible. and will be safe as no wrong assumptions. Sometimes experience can be a death trap.
regarding fuel consumption, if there was a fault in the supply tank gauges (both??) and they remained constantly full on the display (as they should do until usable fuel is taken out of main tank), main tank fuel should decrease according normal in flight fuel consumption. With main tank pump switches OFF there is NO change in fuel level in the main tank.
The pilot could think ALL gauges and/or the CAD or something else in the fuel calc system doesn't work at all. So perhaps ignore the CAD / FMS info and simply continue flying a la clock. (can the CAD be switched off?)
Is that a possible scenario (which Silsoe is just asking us here to understand?) or am I as stupid as me dog. (I still fly with b&w analogue instruments and like them for some reason)