Open a 3-3/8 inch dimension and 2-3/4 inch depth hole, put gauge into it and fastenUsing stepper motor drive pointer, accurate display of the engine speed, and with the LCD display clear display mileage.It takes about 30 seconds to get signal in the first time, but less than 3 seconds after that. It gets quicker under a clear view of the sky. Sometimes if signal is missing, pls. reconnect all wires and restart it.Tachometer fit for Car /Truck/ Boat /crude oil Generator Engine /Gasoline EngineSpeedometer Less power consumption, longer life, higher precision,quick location confirmation, short start up timeSpeedometer: 30MPHVoltage: 12V/24VSize: 85 (mm)Applicable models: GMColour:blackMaterial:stainless steelSize:85mmPackage Contents:1 * 30MPH Speedmeter1 * Chinese and English manual1 * Multi-plug socketOnly the above package content, other products are not included.Note: Light shooting and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.