I’ve finished soldering the face plate for the tachometer and it all appears to work properly.
I wrote some debug code so I could feed it an RPM value through the arduino serial monitor and check the output, it all appears correct.
I’m just waiting on a few more parts to interface it with the car (Optoisolator with a built in schmitt trigger).
I also got a polypropylene sheet to place over the gauge to diffuse the light. I think it turned out pretty alright, for a first test: