I spotted a very high Avg MPG for the same vehicle as mine, when I went in to look at it there were 5 fuel up entries but the first only has a date value and a 0% city value, none of the other fields have any values. Entries 2-5 have values but all of the individual MPG figures are well below the Avg figure.
I suspect the first blank entry is skewing the result?
It could be someone using odometer tracking and that's their first entry. Fuelly doesn't calculate an individual MPG value in some cases. So a combination of first fuel-up followed by partials could look pretty blank.
Keep in mind also that Fuelly calculates average MPG by using total distance driven and total fuel volume filled. It's not an average of calculated averages. So it's possible to have an average that is quite different from individual fuel-ups—especially if several partial fuel-ups are included.
What you could do when you see blank values is click on the "View All Fuel-ups" link under the Fuel Log to double check. That will show you odometer readings.
Had another look at it and the Avg mpg has come back down to normal now. There is one less fuelup entry and the last entry now has extra values in it, litres and price. It is doing odometer tracking like you said, so I guess its possible an entry with an erroneously high odometer reading has been removed?
Could be. I'm not sure which vehicle you're talking about. If you want me to take a look at something and explain it I'd be happy to. You could send it via the Contact Form if you'd rather not bring someone else's vehicle listing into a public conversation.