Was it a Ford? Fords and a few others have the electronic fuel gauges and Avis uses those to determine fuel levels (against their T&Cs). With these, I always snap a pic of the fuel/odometer on the way in & out as I have been incorrectly hit for fuel a handgul of times. Also, unless these electronic gauges have been certified by state divisions of weights & means (or whatever your state calls it), I don't think it's legal to use this for charges.