The best way to figure this out would be to go to a certified automotive mechanic.  It could be the sending unit (i.e. Sensor) it could be the wiring, it could be the motor in the cluster (dash).  Any number of things could be malfunctioning

Rust is caused by oxidation of iron molecules in some forms of steel.  Stainless steel does not contain iron so rust cannot occur.  Water speeds the oxidation process of iron, and salt water exponentially so because of the ions freed by the solution.  Acids do not get rid of rust, however they may clean rust … Read more