Why would you want to do this?. If you must, find a wiring diagram with resistance range on the tank sending unit. I believe these units work with very low resistances, generally less than 20 ohms full scale. The fuel gauge is basically a crude ohmmeter (well not too crude). To increase or decrease readings, either parallel or series resistors will need to be added to the circuit. Also, most of these sending units do not read in a linear fashion. Rather they usually produce a logarithmic resistance, that is as the tank is becoming empty, the resistance that the sending unit produces is higher or lower at one end of the scale and does not drop in a linear fashion as the tank becomes more empty. For example, if a full tank produces 10 ohms and an empty tank produces 0 to 1 ohm, a half a tank will not be 5 ohms. There are also damping capacitances in the circuit to prevent the gauge from bouncing around as the vehicle moves, corners, etc. Again an interesting technical exercise, but what are you looking to achieve??