How to test a factory tach??

I am thinking about a box with two leads to the car battery, and an output to the tach. It would have a push-button to select the RPM. A LED would blink the setting number. A table would be used to equate setting number to RPM for various cylinder engines 4, 6, 8.

For the DIY:
The parts list would include a 5V regulator, LED, MOSFET, inductor, resistors, capacitors, clip leads, prototype board, box, and uC (micro-controller).

If there is interest, perhaps I could source a pre-programmed uC. I would use one with an internal oscillator and calibrate when programming. I am not interested in making units, nor supplying other parts. I would provide a schematic.