Mercury - Speedometer 45 MPH Boat Speed Monitoring Black Face Black Bezel 79-8M6020027. OverviewThis 45-MPH speedometer gives you a steady, no-nonsense look at how the boat is moving. The needle reacts in a calm, predictable way, whether you're easing onto plane or settling into a long cruise. It doesn't jump around when the hull hits a little chop, which makes it easier to judge pace without overthinking it. It feels like a simple, trustworthy gauge you stop noticing because it just works.. Key