I have a 88 firebird 5.0 TBI 700r4. Speedo doesn't work. The tranny tail shaft has a harness with a connecter going to the tail shaft where the speedo gear is. I haven't looked into this whole issue with my car yet but I thought these cars didn't have electric speedos till the later years?

 

They started with electronic speedos around 87. Signal starts at transmission tail shaft. Then goes to ECM. ECM to speed sensor buffer (yellow module either next to ECM or under passenger side panel under map pocket. From there it goes to cruise control module and gauges. If not working, are you getting a check engine light? If not, you are looking after the ECM. If you are, remove the speed sensor on the tranny and check to see if the gear on the shaft is still inline. Had this happen to me on one of my birds. If it slipped, you will need to remove the tail shaft housing and install a speedo gear retainer (will find this at a transmission parts supplier).

 

The Electric Speedo started in 86.

Although; I HAVE seen a couple base 86 Firebirds with a Cable speedo. I've only noticed this on cars that were BASE Firebirds (V6 with NO Gauges cluster ).