Chesapeake White SS 4" Tachometer with Hourmeter - 6000 RPM (Gas - Inboard)4" - Hole size: 3.375" (85mm)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.Benefits:Wide range of scalesMultiple signal sourcesRugged Reliable construction7 character LCD display0-999999.9 HoursUpdates in .1 increments Product : FARIA CHESAPEAKE WHITE SS 4" TACHOMETER HOURMETER 6000 RPM Manufacturer : FARIA BEEDE INSTRUMENTS Manufacturer Part No : 33832 UPC : 759266338326