I just replaced my orig Stewart Warner Tach with a new Gaffrig GL6000W. I think that the rpm's shown are not the same as my old tach, off about 600 rpm. When I installed the tach I reviewed the factory settings, which from the instructions seemed to be set for 8cyl.

I want to recheck these switch positions, does anyone know what they should be for a 420 merc ? I was cleaning up and threw out the instructions.

there should be a sticker on the side of the tach telling you what to setup the switches for.
Also another tid-bit.....some tachs, depending on how old that one is, need a small resistor board because of interfearence from electrical acc. You can call Gaffrig and ask for one.
The new ones should be all ok though.