4″ Tachometer with Hourmeter (4000 RPM) (Diesel) Mech. Takeoff & Var. Ratio Alt Hole Size: 4″ Perimeter-lighted black dial with bold white graphics, black aluminum SAE style bezel, contoured white pointer and flat glass lens. 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 hte 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: Wide range of scales Available for gas or diesel engines Multiple signal sources Anti-scratch glass lens