You are using tripometer tracking. So every time you enter a fuel-up at Fuelly you are telling Fuelly two things: 1.) How far you went from your last fuel-up. 2.) How much fuel it took to get back to full.
With those two pieces of information, Fuelly knows how much fuel you burned to go a specific distance. Fuelly isn't measuring the fuel you just put in the tank, it's letting you know how much fuel you burned since the last time your tank was full. Does that help?
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