I would suggest that anyone with 3 tanks or less get the data thrown out of the calculation. First, most that have 0, 1, or 2 tanks are not being maintained and who knows about the data that was put in originally. Second, usually the first 2-3 tanks are screwed up by dealers, learning curves of new hybrid drivers, and showing off the car to others as to what it can do (acceleration tests, speed tests, etc). Usually if a person has come back 3 or 4 times to enter tank data will mean they will usually continue to come back and keep it updated and be fairly accurate.

I print out every receipt when I fill-up, and write the current vehicle mileage on it ( I have my wife do it for her Pilot as well). I then put them into my overall car database (Ultimate Car Desktop) which maintains not only fuel costs/mpg, but maintenance schedules, repair costs, and everything else to do with the car. I then come here and add the miles driven, number of gallons, and any odd factors that may have skewed the mpg for that tank (4 people in the car with A/C on going uphill through the mountains [both ways ] ). I'm sure many others do the same every fillup, or at least hold onto the data and come here every so often and add 3 or 4 tanks at a time (a hybrid driver might not see a fuel pump for over a month!).

I also think that a date field for the fillup would be good to have to show how often someone is filling up, or how old/stale the data is. This would also give a good indication if someone drives 5 miles to/from work each day and they only fill up once every 2 months, or if they are a high-mileage driver, or if someone took a long trip that had them fillup in 3 days vs their normal 2-3 weeks their tanks usually last.