I’d say as long as your fishfinder accurately tells you the water depth you will be way ahead of the game getting a down speed system like a subtroll, fishhawk or depthraider etc..... it doesn’t mean squat if you’re doing 2.2 top speed of your lures are doing a significantly different speed. Top speed basically means nothing. Also I use the navionics app on my phone to show my course and lake contours, it also shows gps boat speed