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The reason our tachs "drift" up after the power is turned off has to do with the movement. The circuit that drives the "movement" is a frequency to voltage conversion circuit. It drives the twin coils of the tach movement in opposition to give the appropriate needle reading for the speed of the engine. When power is cut various components discharge at slightly different rates (capacitors) and cause the two coils to be unbalanced, thus the needle drifts. Newer cars which use digital movements have a self zero (for lack of a better description) feature which "parks" the movement at zero when the power is killed.
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