[Christ]

The problem with using a 4 cylinder tach in a 3 cylinder is that the tach reads output from the ignitor and does math.. x/4 = RPM. Lets say x is 4000 on the 4 cylinder, your formula is

4000/4 = 1000 RPM (idle... )

Now, lets assume you put that cluster in your car, and don't change anything about it.

4000/4 = 1000RPM for a four cylinder,
but for the 3 cylinder, the actual formula should be:
4000/3 = 1333.33

So basically, your engine is revving 1330 RPM when your tach is displaying 1000 RPM. The difference won't be linear either... it's a curve.

16000/4 = 4000, 16000/3 = 5333.33.

Now, if you can change caps or resistors to make it do the math correctly, then you're set to go, but just using it without modification will definitely yield an incorrect reading.