Bad gas mileage

Yes, easy to make errors calculating mileage.
By brother used to use the "I get *** miles out of a tank, I think I have a YY gal tank, therefore I get ZZ MPG" method, and he was absolutely WRONG.

You gotta know tha actual miles traveled and the actual gallons used.
WRITE IT DOWN. Don't try to remember it.
Better to do it about 3 times to avoid anomalies.