Hi All! I changed out the tach in the Blue Car due to it bouncing. The spare I have doesn't bounce but it reads lower than the one originally in the car. The first one read 4k @ 80mph. The new one reads 3300 @ 80mph.

In your experiences, which is right?

It's a stock 72 1.7 with 205/60x15s. Dunlop A2's, if that matters.

about 3800 @ 80 mph ...

in my experience, experience is useless in the presence of instrumentation.
find yourself a calibrated tach and see how far off the stable one is.

Use the rpm setting on your dwell meter to check it. Remember that your speedo will be slightly off due to the tire size too.