Look around the outside of the gauge, most usually have a list for the switch setting and # of cylinders on a sticker. If it is made by Faria for a Mercruiser, then setting 1 is for a four cylinder, 2 is for a six cylinder, and 3 is for an eight cylinder engine. Use a small screwdriver and push that screw in slightly, and set it to #2.
The switch on the tach tells it how many pulses per revolution to count in order to provide the correct reading. Your old 3.0 only fired twice per revolution, whereas your 4.3 is firing 3 times per revolution. GA_Boater has the math correct and this reflects in your tach reading.
My neighbor had the opposite problem with his 4.3 when he replaced his broken tach. Had it set to the 8 cylinder setting and his tach read low. Put it on setting #2 and all is well with his boat.
