Good question and I can't answer it. I do have a Humminbird Matrix 37 with the speed wheel, and also a Garmin GPS. Neither is much good under 1 MPH, and some of the differences between a paddle wheel and GPS can be caused by currents. The paddle wheel gives you your speed over water, and the GPS your speed over land. If you were fishing a river with a 2 MPH current and heading upstream at 1 MPH, your paddle wheel would indicate 3 MPH and your GPS would indicate 1 MPH. In reality, for fishing, the paddle wheel speed would be the most important to you as far as bait or lure action.