I bought a 1981 Z28 recently. The in-dash tach reads double what I think the engine is really turning. What could be causing this?

Might could be something in the tach wiring. Bad power/ground, bad filter on the wire set, stuff like that. It's entirely possible that the tach itself is bad.

@galyonj I have yet to do the basic tune up so I hope that does the trick. We'll see.

The (factory) wiring in my C5 (which is 22 years newer than your Z28) is a bloodbath that I'm still working through. Rusted grounds, brittle insulation, bad/burnt sockets, burnt resistors in the interior controls, the whole bit.

 

I don't say this to try to turn you off of your project, but to prepare you for what you might encounter as you tuck into the project.

 

If I were you, the very first thing I'd do is take a dremel with a wire wheel to every single ground I could find in that car. Get all the rust off them, make sure the connections are tight, and then reevaluate. Electrical gremlins will ride you into the ground if you let them.

2nd gen F-bodies are rad. Be methodical, plan the work, and you'll be fine.

Is it stock?  Aftermarket distributor by chance?  stock tach?  Aftermarket ones have a switch for pulses per rev.

23 minutes ago, Deephaven said:

Is it stock?  Aftermarket distributor by chance?  stock tach?  Aftermarket ones have a switch for pulses per rev.

It's in the shop right now. But I suspect the distributor is aftermarket. At lease the cap is. The tach is the original one in the dash.

 

Bet it is just wired wrong then.