I have a 91 camaro RS. It has the factory 3.08 non posi .
I want to change to a 3.23 or a 3:42 posi but I have to ask.. What problems would I run into? I have heard it screws up your speedometer. Is this true?

Changing to another 10-bolt (or 9-bolt) rear end will be an easy, direct bolt up.

If you change the gear ratio it will make the speedometer be off(there's calculators online to determine how much). I would think that changing to a 3.23 gear wouldn't be that drastic of an error on your speedometer though.

but it will cause some change

Red is correct, your speedo will be off. There are calculators to determine what gears you need to put in the sender for your speedo to read correctly, based off of rear end gears and tire size. As long as you are swapping a drum brake to a drum brake rear, or a disk brake to disk brake rear, it's easy. Even switching from drum to disk is fairly easy.

One suggestion, I don't know if you already have them, but as long as your rear is out, switch out the control arms and the panhard bar.

Yes, but the fix is easy. Check the T-5 sticky at the top of the page.

JamesC