hmm, I'm not sure offhand why you'd want to ignore folks who aren't fueling up anymore. Isn't their fuel economy data just as valid as people who are fueling up today? How would ignoring them help you compare?
No, in many cases those "one tank wonders" are not valid and should be ignored. I find, looking at sites like fueleconomy.gov, that many people will post stellar mileage early in their cars' lives and not continue to report honestly as fuel economy normalizes. For example, one 2007 Nissan Versa owner reported one tank at the end of 2006 at 36.6 miles per gallon and then he disappeared. Was his mileage mis-reported and he failed to report the 25 mpg he probably got on tank #2 or was he a plant from Nissan? We'll never know, but knowing that date gives us a good reason to ignore that figure.
Comparing to similar cars that are still being reported is very useful as our cars age and things gradually change.
Well, we only show cars that have three or more fuel-ups on the public pages. You're right that one or two fuel-ups doesn't tell us very much about a particular vehicle. I'm just not sure that things gradually change enough over time that we should ignore a vehicle with a good number of fuel-ups just because there isn't recent data.
By adding a Date of Last Fuel up it would be up to the user to determine if he/she would like to ignore old data. Being a Hybrid driver I was really disappointed with my mileage as I bought it in the Winter. My numbers are coming up now so seeing current fuel ups gives me a better idea of how I am doing.