It is just the easiest way, and likely only way for most people, to determine how much gas they used since the last calculation. Otherwise, you need a way of measuring the amount of fuel left in the tank, and keep painstaking records of how much fuel was added and left in the tank. Or have some way to measure the fuel that goes into engine.

For those that would be doing partial fills most of the time, the best thing they can do is do a handful of complete fills while also recording the cars average fuel consumption display, and from that calculate a correction factor for the display. Then they can get a reasonable reading from that during the stretches between full fills.

Yes I understand it's the only way to get a accurate figures, for me it's not a problem as I manage my finances and make sure I have enough money to fill up. But I don't know many people that do here, unless doing a long trip, many people hate the thought of spending too much at once, and only put small amounts in at a time.

The simple and obvious solution - fill the tank, no matter how painful and expensive it might be. Then, when the gauge gets down to 3/4, fill it again. Each fill will cost no more than it did before, and you will get reasonably accurate results. We who live in climates where winters are severe learn to never let the tank get below 1/2, just for safety reasons.

There are plenty of reasons to carry a full tank, and there are plenty of reasons not to full up. But ultimately, if somethings expensive, whether it's food, alcohol, cigarettes or fuel, people would rather buy smaller amounts and make it last longer. In a small car, you can really feel the difference in performance with a full tank V's an almost empty tank.

It's not a criticism, I like using fuelly and it works perfectly for me, but it only works well if you start and end with a full tank.

as long as you keep recording the fuel used and the mileage done it will work out in the end.You just need a "reference point" and for most people "full" is the easiest.If you have enough money to live from week to week i dont see the issue with filling the tank.If nothing else you save some time by not going to the gas station every few days.
As an aside it says that you can either record odometer or trip meter but i dont see that option?

You can change your tracking preference by clicking Edit Vehicle in your car's profile (by the image).

Sorry to bump such an old thread but I have a question. I, like many of you, live in a cold climate so I try to keep my tank about half. When I fill my tank, I fill it from half tank to full tank, record the odometer reading, etc. Will this app still work accurately even though I am not usually filling the tank from empty to full?

That's fine.
A full tank is one that is filled up. Doesn't matter what the amount of fuel was in there to start.

A partial fill is when you don't add enough fuel to fill the tank. Let's say, take it from half to three quarters. A tank fuel economy can't be calculated because to fuel added is less than what was used, but the data can used for the vehicle average once another full tank is done.

So basically as long as I always fill the tank completely, Fuelly will work accurately?

Yes, you could fill the tank, drive 6 miles, fill back up, that's a FULL FUEL UP, and fuelly can calculate your economy just fine. Distance is not important, but filling up to the top is. A partial fuel up is anything less than full.

You'll see a lot of plug in/hybrid owners only putting small amounts in, probably trying to convince people their cars are more efficient than they actually are by only driving small distance and then filling up again ;)