the full/empty thing really works for the rent companies if you have the following experience. This has happened to me twice so I do not think it is an accident. I rented a small Diahatsu from Hertz in Barcelona When I was taking it back the fuel indicator quickly went from one bar to empty indicator light. I put in a fiver worth but it did not make any impact on the indicator gauge so I put another fiver thinking it must have been empty. A few miles down the road the indicator bars suddenly jumps up to quarter full which is the way I gave it back.