There adjustments on many tachs. The first gets it in sync with whatever sending system you have, then a tiny potentiometer to fine tune them.
My old VDOs were no longer taking an adjustment so I replaced them with new Farias.
One easily adjusted, the other I couldnt seen to turn the potentiometer and with a little harder push, the screwdiver punch right through. Back to warranty.
So be careful trying to adjust any of them.
But first...check all your connections in the positive, negative and signal wires.