Take the float out of the tank with the wire still connected. Attach a ground wire to the mounting flange (you can use a small bolt and nut to do this). Then move the float up and down while someone monitors the gauge. If it reads full when the float is down, and empty when it's up, and it goes up and down smoothly as you manipulate the float, then you've confirmed that the float is the problem and not something else. Once you've confirmed that, figure out how to reverse the float, as LeeP points out, so that the resistance increases as the gas level increases.
It's a very simple circuit. The flange is grounded, and the wiper bar that moves on the resistor adds resistance to the circuit based on the float level. When the float is in the down position (empty), the wiper bar should be all the way at the bottom (or top, depending upon its design) of the resistor. If, when the float is down, the bar is farthest away from the grounded side, the float needs to be reversed.
