How do you figure that? Nowhere does Fuelly ask for tank capacity. It asks only for fuel consumed. If you go 75 miles and burn 2 gallons, it will report 37.5 mpg. It doesn't matter whether the fuel tank holds 3 gallons or 30.
Fuelly only asks me to report how much gas I put in. Without knowing what the total capacity is, how can it tell how much gas I burned?
The only way to tell is to take total tank capacity and subtract from it how much gas I just added to fill it up full. Alternatively it can assume that however much gas I put in in the first fill up is the total capacity but I've tried arbitrarily changing that and it doesn't affect any stats. Then the conclusion is that Fuelly must have a database of how big each car/bike's tank capacity is.
>Then the conclusion is that Fuelly must have a database of how big each car/bike's tank capacity is ...
>... 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.
You've got it all wrong. Fill up, go use some fuel, fill up again and the figure you see on the pump, be it 2 gallons or 10, is the fuel you've just used to do the miles you've just done. You have to brim it each time to get accurate figures. Get it?