Forestg may be right, it sounds like your tach is just half speed. Fiat 850 has the same tach...well, as nearly as I can tell, and I can only guess that the tuning of the variable resistor makes a difference for 4 or 8 cylinder. But as long as you are connected to the coil, there is no need for a "tach amp" (taking the big inductive kick to detect a fire). You would need a tach amp if the drive were "digital" (of just 0-12v). If only I would have known that when I put the Electromotive ignition on my 308...
Some Speedo shops (Palo Alto?) have standard circuit boards that replace the Veglia printed circuit board and work from either the digital input or the coil based pickup (see pic). That Veglia circuit is simple, and about the only thing that can fail is a capacitor--which are just pennies to buy. See the components with cracks in them--capacitors age and fail.
One of the projects I have no reason to finish is taking a 8krpm stepper motor Tach (about $30) and installing it in the Veglia can... but if you have patience to gently remove the bezel ring, I would try to wiggle that variable resistor there at 12:00 in the second picture...Lee
