Tachometer off

I recently bought a 90 MJ. The previous owner put in a set of guages that has a tachometer in it. The problem is that it reads way low. At Idle its about -200 RPMs. It doesn't have a little needle at 0 (never did) like my buddy's 96 XJ does.

Anyone got any ideas how to make it work right?

Is the MJ a 2.5L?

No, 4.0 Renix