The fuel sending unit is basicly a voltage regulator and your gas gauge is none the less, a volt meter( but reads small amounts of voltage ), on older vehicles, tanks had floats in them, the float was on an arm, attached to the sending unit, the float goes down, the sending unit decreases the voltage to the gauge, if the sending unit is decreasing the voltage wrong ( or increasing it instead ) it will do things like that. If you can figure out how much gas you have by seeing patterns in the gauge readings, that's fine, it's not gonna matter if that gauge works or not but, until you can get it fixed or see the patterns, leave a full 5 gallon gas can in the back so you can get to a station if you run out.