I have an '85 fiero gt I just purchased. Nothing on the instrument cluster worked at all when I first got it, now after replacing all fuses the speedo works, the tripometer doesn't, the odometer works, not sure if the fuel gauge does, oil gauge goes everywhere, the weird thing is the tachometer slowly rises higher and higher extremely slowly. To the point where at about 10 min the tach spun in almost a complete circle stopping at the start... any idea what is causing this and how to fix all my gauges but specifically the tach.

It is probably your tach filter. It is located near the coil.

Arn

Do I need to replace it?

Yes. Either way, it will have lost it's value after all these years and will not be reading correctly anyway. It is a cheap fix

Arn

Thank you!

I replaced the tach filter, now I have a new strange issue. When I turn the key to the ready position but dont actually crank it, the tach goes to 500 each time it cranks it ads 300 to 500 to the tach then when it starts it spins back to the start again. Any idea what to check next?