MPG
The driver is a large determining factor. This is why the woman who sued over low mpg doesn't really have a case. I get substantialy better mpg than another member of my family, driving the same vehicle.
In the old days, there was this thing called the Mobil Economy Run. The secret, besides over-inflating your tires by a few psi, was "driving as if there was a raw egg between your foot and the accelerator pedal." coasting, keeping highway speeds down, etc.