Since I believe that the Nautiue Logic unit is mimicing a paddlewheel signal but with the GPS being the source of the input, it doesn't surprise me that your readout would be the same. Under the dash, probably over by the ignition should be a little dial knob. that allows you to adjust your speedo reading. go out and use your app again and turn the dial until the speedo reads the same as the GPS on your app, then you should be good to go.