"If you don't want to track the miles between fuel-ups yourself, you can simply enter your odometer reading each time you fuel-up. You won't get any stats on your first fuel-up, but Fuelly will begin tracking your miles (or kilometers) from then on."
We measure your fuel economy only after we know how far you have traveled and how much fuel you have purchased for that. Your first fuel up with odometer tracking gives us no reference distance. It's unfortunate (but necessary) that your first fuelup doesn't give you an economy number but it starts working from the second fuel up onwards, and it's much easier to track versus doing tripometer tracking.
Looks like you found a bug in our system that we need to patch up. Take a look at your fuel log, and you'll see that every entry has "odometer" except the first one which has "miles". If we can figure out how that happened, we can get it patched up—because we shouldn't allow that to happen.
Can you give us a little more info about how that happened? We don't allow zero as a valid odometer reading.