When do the last time (if ever?) that the pumps in town were calibrated??? That is, calibrated to read ACCURATELY? In the US for example, the NIST can shut down any pump/station if a pump measures a discrepancy of anything over 3.3 ounces per 5 gallons pumped.
Diane, what you are saying HAS to be more than a calibration issue, but you can't imagine HOW many times the gas station has been able to cram, say 13 gallons into our 12 gallon tank. It's truly amazing.
Stilromin: your solution avoids certain fraud, but not if the pumps are off to begin with.
What to do? Bring a clear 5 gallon container and say "fill it up to this line." THEN pour it into your tank...??? I dunno...