What's going on with my tachometer?

[Vette66AirCoupe]

The tach in my '66 427/390 seems to think the engine is turning much slower than it actually is. I recently purchased this car and have been shifting easily without winding it too tight. Today I noticed that in 4th gear doing 40 MPH the tach indicated 1,000 RPM's. The car is equipped with an M-21 close ratio 4 speed and the rear gear ratio is 3.40:1. I wound it up a bit and the tach was reluctant to pass 2,700 RPM's while the the driver, me, was reluctant to wind it any tighter not knowing if it was actually approaching red line RPM's. I plan to get it checked with a tach and dwell meter to see what I can find out but I'm guessing I'll find that the tach is indicating way less RPM's than the reality of the situation. What's the likely culprit? Does anybody have any ideas as to how I can diagnose the problem? Thanks in advance for any suggestions you can offer.

 

[JohnZ]

The tach is driven by the distributor at half engine crankshaft speed (two engine revolutions = one distributor shaft revolution), and the tach mechanism converts that to crankshaft rpm. Several possible causes for your symptoms - drive cable "square" ends rounded off, caked-up old lube in the cable, distributor mainshaft drive gear and/or cross gear worn, tach head cup magnets weak, etc.

If you disconnect the tach cable at the distributor and run it with an electric drill and the tach responds appropriately, the problem is most likely in the mainshaft or cross gear teeth. If it shows the same symptoms you see when driving it, it's probably tach rebuild time.