Abstract: A system for metering a fuel additive into a flow of fuel into a fuel tank includes a mixing unit coupled to the filler neck of the fuel tank. The mixing unit has a body with an axial main flow passage, having a fuel inlet and a mixture outlet. The fuel inlet is dimensioned to receive the standard nozzle found on conventional service station pumps for motor fuel. The body also has an additive inlet that communicates with the main flow passage through a metering passage controlled by a metering valve. The metering valve is actuated by a pivot arm having a first end that is in the flow path of fuel flowing through the main flow passage and a second end that engages one end of a pushrod, the other end of which engages a valving element in the metering valve. The flow of fuel against the first end of the pivot arm causes the pivot arm to pivot so as to push the pushrod against the valving element to move the valving element away from its seat, against the force of a biasing spring, to open the metering valve.