You could just drive until it runs out then call up AAA and ask them to fill you all the way up and see how much it takes. But i am sure that the computer isn't stupid when it calculates the mileage and gas consumption. Otherwise ford would have gotten rid of it.
You also have to take into consideration that your engine lets say 4.0 for the sake of arguement. That 4.0 has gas inside of it, so there are many variables to take into consideration.
the key is to try and not go to the cheapy gas stations cause you run a higher risk of getting watered down gas.
2007 Mustang GT Convertible Stage 0 E-Force #402 Supercharger, Brenspeed Tune (435/400), GT500 Fuel pump, Clutch Delete Option, Roush Front Fascia, Brembos, California Dream Hood, Qwitch latch flush mount hood pins, MRT Light Bar, J&M LCA's, BMR UCA, UPR catch can, 6000k, Pypes Bombs, Pypes X-pipe, Airbrushed Flame pieces under the hood, Clear corners, GT500 shocks , FRPP P-springs (rear), BMR Adj Panhard, E-Force engine covers
2012 Ford Focus SEL 5Dr with MyFordTouch, Auto-Park, 303A Package,