4" Tachometer (6000 RPM) (Gas Inboard & I/O)Perimeter-lighted Silver dial with bold Black graphics, polished stainless steel bezel, contoured Black pointer, and domed glass lens.Electronic tachometers work by counting pulses generated by the ignition 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 position you let the tachometer know how many pulses are sent per each engine revolution.4" - Hole Size: 3.375" (85 mm) Specifications:Domed, Scratch Resistant LensStainless Steel BezelLED Illuminating around the Perimeter of the DialFactory P/N TC9476*Sold as an Individual**Gauge Cover not Included