This invention is directed toward a computer-controlled auxiliary fuel tank system that works with both gas and diesel-fueled vehicles, and can operate independently, or in combination with an auxiliary fuel tank, auxiliary fuel pump, sending unit, check valves, inlet and outlet fuel lines, wiring harness, computer module, auxiliary emissions canister assembly, vehicle installation means, and LCD and LED display devices. The invention is not dependent upon any other computer systems and is fully transparent to any on-board systems. In addition to overseeing the transfer of fuel from the auxiliary tank to the OEM tank, the fuel monitoring system (FMS) monitors a variety of functions of the vehicle, and from that data can calculate and display a wide range of information for the user, and is fully user calibratable. The invention not only warns of low fuel situations, but also provides a wide range of diagnostic tools to analyze and display problems with the fuel system.