Fuel Gauge Accuracy

My fuel gauge in the '65 Valiant Signet convertible started working again. When the needle was exactly sitting on the empty line I filled up. The pump clicked off and the attendant asked be if that was good? I asked her to fill till it clicked off a second time. She pumped some more, the pump clicked off, and (of course) a little fuel shot out onto the side of the quarter panel. With the tank now full, the gauge actually is reading overfull (past the full line on the gauge).

I paid for 58.335 liters which is the equivalent of 15.4 gallons.
18 minus 15.4 would mean we have 2.6 gallons left in the tank when the gauge needle first reaches the line marking empty. Feel free to contribute your own findings. Thank you.