I have an 89 coupe that has more problems than I can list. The one problem that confuses me is the tach. It reads about twice the rpms that the motor is really running. If you have any ideas please let me know.
i know aftermarket tachs have a switch on the back (most do) to select number of cylinders. as i recall, i think the oem ones do to. i would ask someone to confirm that first before you act upon it. might be a tach out of a 4 banger. my best guess.
good luck.