Gauges generally have only four connections. 1) +12 volts when the key is in the RUN position. 2) ground, 3) internal light fed from the NAV/ANC switch. and 4) a sender signal for fuel, tach, oil pressure, engine temperature, and trim gauges. A voltmeter has only three connections since there is no need for a sender -- it measures system voltage. The speedometer has no electrical connections other than the light since it operates from a pitot at the back of the boat or from the pitot built into the outboard lower unit or an I/O outdrive. The gauges will come with complete wiring instructions. You will need to order a gauge set for either an outboard or an inboard/I/O.