Real MPG gives a Gaussian average of every type of measurement. Least accurate is from the car's computer. Next least accurate is brim to brims based on the car's odometer. Most accurate it brim to brim based on a corrected algorithm of the car's mileometer. But most people judge it against their perceived mpg based on the odometer, so a Gaussian average of everyone's figures measured every way for every type of driving is the most relevant figure.