Eratic dash

I have a 1995 vintage Infinity G20 that the Tach and speedometer needles seem to jump around on occasion. Does anyone know what might cause this?

The two devices might share a ground connection. If they do, and this happens with both at the same time, it is probably the problem.

I should have said the tach is almost never right and the speedometer is right 95% of the time.

How do you know the tach is “almost never right?”

The tach often reads 5000 rpms when idleing. Sometimes it reads 0. After driving for a while it peggs at the redline. I never cared much about it but now I am trying to sell it.