I have had situations where the pump malfunctioned somehow, and auto-clicked off after putting well over 12 gallons into my 11.8 gallon tank -- and I was nowhere near empty. So my mileage for that trip was very poor.

At my next fillup, lo and behold I got absolutely tremendous mileage.

My interpretation is that the pump and it's autostop mechanism malfunctioned. It was one of those pumps you get sometimes where the gas just barely trickles into the tank, very slowly. And it just keeps right on filling, with the autostop mechanism not getting triggered until the tank was overfull.

I've had this happen to me twice, and both times, the subsequent mileage at the next tank of gas (calculating by hand) was well beyond what I should have received.