4" Spun Silver Tachometer with Hourmeter 6000 RPM - Gas - InboardElectronic 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.Benefits:Wide range of scalesMultiple signal sourcesRugged, reliable constructionSpun Silver4" Hole Size