The gauge calibration is controled by what we used to refer to a ballast resistor within the gauge circuit. Now I think it is located near the flasher unit on the peddel box. Small rectanglular alloy box with a couple of wires on it. I know that on my old Fords if these failed they would make the gauges read incorectly though I have never heard of your situation. I would try changing that unit first. I hope someone else can give you the correct name for it. I hope you don't have a problem with the circuit board as they can be expensive to replace or repair