Calibrating tachometers?

Full wave rectify it to get 120Hz for 1800 RPM. Danger !!! That circuit is line powered, isolation transformer and fuse advised.

To get most tachometers to trigger you need 60+ volts, so the sound card idea will not work. Unless you run an amplifier and transformer. Beware most amps will not like the high inductance of the transformer and might be damaged.

I program a micro controller and use a MOSFET and inductor to simulate ignition from a 12V supply. The inductor boosts the voltage, like is done on the primary of a standard ignition. The micro has a crystal time base so it is easy to get accuracy to 50 parts per million. This feature is built in to my engine management systems for waste spark systems, since the spark is distributed from multiple coils. While my system displays RPM via USB on a PC, the tach drive fires for every ignition event, enabling the tach in the car to work.