I have been noticing in a lot of pics that the 89-91 hatch has a tach in the cluster. Mine however does not. Was this a factory option or are people adding them from another car?

 

Si trim civics and CRXs came equipped with stock tach gauges in the cluster. 88-89 are different than 90-91. I'm assuming you have a 91 civic, so grab a 90-91 civic si or crx si gauge cluster and bolt it in.

Doesn't hurt to read the FAQ thread stickied at the top of the sub-forum either.

 

Pretty much nailed it. It is a factory option. Very easy to swap also. Ben Ogles site, EF-something, has a decent write up on swapping the clusters. Easy to find on google.