Abstract: In accordance with the present invention, the actual volume of a liquid chemical additive used to treat fuel is measured by an additive flow transducer so that a desired mix ratio can be maintained. In a first preferred embodiment, the amount of additive measured is converted to an equivalent volume treated fuel, and displayed. The user watches the display and stops adding additive when the display indicates that he has already added sufficient additive to treat the actual amount of fuel to be added to the fuel tank. In a second preferred embodiment, the user sets in the desired mix ratio and the volume of fuel that is to be added to the tank, and an electrically driven pump adds additive to the tank until a desired mix ratio obtains. In a third preferred embodiment, both the cumulative fuel flow and cumulative additive flow are sensed and the rate of additive flow is controlled by an automatic control system seeks to maintain a desired mix ratio.