tried searching? Tacho Showing Over 9000?! - Skylines Australia
mines started doing this too. give the dash a bash when it happens, if it bounces around when you hit it, you know its just some connection or resistor in the tacho playing up. apparently theres also a calibration screw on the back of the tacho and giving it a few turns back and forth to loosen it up can help.
or take the tacho out and send it a gauge place, theyd be able to fix it.