Get a potentiometer (pot)(variable resistor) at Radio Shack rated at 100 Ohms. Install in series (inline)some place on the sending wire. Hook Terminal 1 and 2 together and to one end of sender wire, terminal 3 to other. Adjust pot until gauge reads what's in the tank. Apply a drop of "glue/silicone/etc" to adjustment screw to lock it.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn