If you're not using your DIC to tell you fuel consumed, wait until your next fill up to report your fuel consumption. There's a lot of variability in the gas gauge. You may have consumed 3 or maybe 7 gallons at that point. When refilling your tank, use the same pump as last time, and fill only until it clicks off... do not fill further, and you should get a fairly accurate consumption measurement.
Oh wait, you're in the land of Nazi gas stations, right? Well, at least tell the attendant not to fill past the click.