I just bought a 90 Camaro RS, sadly the speedo seems to be out by about 60kmh, makes for kinda interesting driving in town, what are the possible causes, and how the heck do I fix this.

Before starting, you should find out what the car was OE equipped to rear axle ratio and tire size. Find and read RPO sticker, on console door(?) for ratio, GXX code, and tire label for tire size. Then find out what ratio is installed now and present tire size. GM sells a speedo cal chip package for the back of the inst cluster.

Remove the cluster, attach supplied harness to inst cluster, power the harness momentarily across battery to 'burn out' circuit board solder land. Then, with a supplied chip, cut off x number of legs on chip, according to chart supplied listing rear axle ratio verses tire size. Plug in chip into open inst cluster socket and reassemble dash. Read the instructions carefully to not cut off the incorrect legs. Instructions are god as how to do this action.

If you remove the inst cluster and find a chip already installed, somebody has been here before. Then as said, cut legs and install chip after removing present chip.

Welcome to the club!