Mpg

Hi
I have a A8 2.8ltr but would like the 4.2 but i have an issue with economy as i have read that the 4.2 is just as economical as the 2.8ltr yet some say this is not possible, so i am now putting the question to the ones that know, that is the forum members as you are the true judge being the owners of the aforementioned vehicles.
Many thanks for all replies