It doesn't matter about US/Imperial gallons, we all have 85 litre tanks. The problem is possibly that the indicator is neither calibrated nor linear nor does it go from 0 to 80 litres - all that considered the way to check is to compare fills (fill up with the same amount next time at a different station and see where the needle points). Saying that a Tesco station was short 'changing' customers for weeks before it was discovered.

Aren't pumps calibrated by weights and measures - if calibration seals are broken they're in big trouble