Fuelly calculates Avg MPG by taking the total miles tracked divided by total gallons tracked. That's all there is to it.
Looks like your case has a wrench or two in it because you have some missed fuel-ups. So Fuelly might not be able to get a completely accurate totals miles tracked, and that could be throwing things off a little. I'm guessing once you have three or four more fuel-ups in a row that aren't missed you'll get a more accurate average MPG.
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