IMO, I believe the vehicle gage. The manual needs ti be more than gallons in and reading the miles off the odometer. The gallons in is a big variable IMO. Personally I know that I can drive 5oo miles and vary how much goes in anywhere I want. The indivual hand nozzles cut off differently from one to another and the same one varies on day to day and how fast the fuesl is going in. Do you top off or over fill and not know it. I trust the onboad readout much more than manual calculations and it is usually close but read less than I get manually sometimes.

The you need gas is the mfg worrying about the **** lawyers again. Get what if you are half way to Calf crossing the desert and the next station is 127 miles away? that sort of thinking. It's ok to have it but I think we all do our own thing and buy gas when we want or need to.