Well I got excited and hacked up my rev counter... before I have tested it.
It should theoretically work, but I've probably made a mistake somewhere!
anyway what I've done so far was very simple. I cut some slots in the face surround so I could rotate it 90 degrees, but that was self explanatory. It allows me to mount it to the vertical face of the instrument cluster surround which is the only place I have space for it. It meant I had to dremel a new slot for the cylinder selection switch.
I figured out that the 8 cylinder setting has a resistance of around 400 kΩ and the 4 cylinder setting has around 200kΩ, so I bought a 100kΩ potentiometer and wired it in place of the 400kΩ resistor.
I chose a variable resistor to allow for any slight error in my extrapolation. Should be able to use it to calibrate the tach for my engine and get a more accurate reading.
Simple!
hopefully it will work