The heavy-duty automotive 12 volt DC Voltmeter is the perfect gauge for vehicles with 12 volt negative ground electical systems. Fits a standard 2 inch mounting hole with bracket provided. Note: We recommend wiring this unit to the ignition to avoid any battery drain from the backlight when the vehicle is not running. Additional Details ------------------------------ Product description: The heavy-duty automotive 12 volt DC Voltmeter is the perfect gauge for vehicles with 12 volt negative ground electical systems. Fits a standard 2 inch mounting hole with bracket provided. Note: We recommend wiring this unit to the ignition to avoid any battery drain from the backlight when the vehicle is not running.