The vehicle computer samples the pulses of the fuel injectors and calculates the fuel delivered based on the amount of time the fuel injectors are open and delivering fuel. Strange thing about that - if your vehicle is able to show the instantaneous mpg, if your fuel pump is going bad and not delivering fuel, the instantaneous mpg reading will go really low because the engine computer is calling for fuel and the injectors are being held open. By the way, all vehicles seem to be optimistic in determining their mpg.
