>Then the conclusion is that Fuelly must have a database of how big each car/bike's tank capacity is ...
Nope.
>... Without knowing what the total capacity is, how can it tell how much gas I burned? ...
The gas that you put in, is the gas that you burned. I.e., you start with a full tank of whatever-gallons. You drive 75 miles. You fill it up again with 2 gallons. Hence you drove 75 miles on 2 gallons.
|