Abstract: A safe, reliable system for automatically dosing diesel truck fuel tanks with a fuel additive is provided. The additive, such as a concentrated solution containing fuel borne catalyst (FBC), is fed by gravity and dosing is controlled with a signal from a switch located on the ignition switch and/or the fuel tank cap and filler spout. When the ignition switch is in the on position and/or the cap is removed, a solenoid opens a valve and permits a timed or otherwise measured amount of additive to flow into the tank while the cap is off. Replacing the cap and/or turning off the ignition switch preferably stops the flow of additive, the flow rate being set to supply a predetermined amount of additive during an average refueling interval.