Electronic tachometers work by counting pulses generated by the ingition system, alternator, tach signal generator or magnetic pickup sender. The tach is hooked up to 12VDC, Ground and one of the signal sources listed above. By selecting the right tach and setting the switch on the back to the correct postion you let the tachometer know how many pulses are sent per each engine revolution.Features. Color - White. Size - 4 in.. Material - Stainless Steel. Hole size - 3.375 in.. 6,000 RPM Gas - Inboard I/OBenefits. Wide range of scales. Multiple signal sources. Rugged, Reliable construction