I know the signal for my '73 comes straight from the ignition points so you basically get a pulse per cylinder which is how I think they all are. Are you sure the unit worked ok before you fitted it.
Mine has been bad since I got the car and it does exactly what you describe, it is ok at idle but only shows less that 1/2 the true rpm at higher revs.
There must be an off the shelf electronic module that can replace these with a stepper motor to drive the needle or similar....I've yet to see one though.