The 91 and up tachs have a selectable switch on them, at least all the ones I've worked on. On the earlier ones, the pot I believe is for adjusting full scale meter deflection, and the single resistor (IIRC) on the board can be changed to switch between 4/6 cylinders. You could get a similar value resistor and alligator clip it across the resistor on the board, effectively halving it's value, and see how it affects the meter movement.