[ecomodded]

Mustang you got to look closer at why your mileage is fluctuating, your driving it, or someone is siphoning your gas on a semi regular basis..
No one but yourself can figure it out.
what a question really, and with no background info you want the right answer from a blank equation, its just not possible, your the one with the info, your driving the car in question,how can anyone possible know but yourself, which you happen to not know so nobody will know until you figure it out and tell us. Start keeping track on paper of highway, city miles, and speeds driving, fast highway sucks gas slow highway does not for instance do a mental note if your able and gain some understanding as to how your driving affects your mpg, it is you or that gas thief who is responsible for any fluctuations in your gas mileage.