How about instead of top users, top make & model year by MPG? (min of 10 vehicles w/ 10 fillups each, so if someone has a 50 MPG 82 DeLorean (, it won't top the list because no one has that make and M.Y.)
And before someone gets on me about a 50 MPG DeLorean, take one to the top of a hill, use a gas can to top it off. Engine on coast it down hill to the station and top it off again. 0.2 miles travelled and 0.004 gallons used does yield 50 MPG. Unlikely, yes, but if someone wanted to "cheat" like this, it is possible.
Well we currently have a list of the top ten most registered vehicles reported at Fuelly, but not by MPG. Though it would be nice to see an average MPG next to each one. I can't see them setting that up, as its still something that someone would try to get on top of... It sucks but it happens.
Then Gen 1 Honda Insight Hybrid with a manual transmission is the king of high MPG vehicles if that is all your looking for. I would suggest looking there if you want to see what actual drivers are getting.