It's been written before, but one of the many reasons we usually rely on trip computers to report "real" fuel economy is that we have no control on who filled up before us, how and when. Different pump nozzles won't "click" at the same point. Did the other guy stop at one click, or two? Sometimes you get pumps with gag reflexes and you overfill. Or underfill. Also, no control on how much driving the other guy did between the fill-up and the drop-off, as there are multiple gas stations all around. If you fill-up at Costco and drive 15 km, you're still "full". And who knows if a fleet guy made a coffee run?
We're often capped at 400 km for weekly loans, so it's not like we run multiple tanks over seven days - unless roadtrip - to help with the data. Sometimes I get a car that's not full, so in that case I fill-it up (and get $crewed). If testing a PHEV, I fill it up after pick-up, sometimes adding barely 1$ in fuel, because they use so little I need a proper "reset" of the tank. Making that detour often plays with time, so for regular vehicles, I used to do it but don't bother anymore.
Previous pressie went 12.1 indicated, 12.5 math so tank was pretty full. The bigger the tank, the more "Full" can trick you. A truck will be full for a long time. The truck I dropped yesterday indicated 12.2, math was 13.5 - not full, or optimistic computer ?
Usually, the longer you drive the same vehicle, the more your fuel stats - both math and computer - converge towards a similar number. So my numbers-OCD self calmed down and reports what the car says - I can't control all the variables. Some guys just quote Energuide
