I use Lowrance so I can't tell you jack about Humminbird. However, I can tell you that on my Lowrance Elite 5 DSI, my oldest unit, that the GPS only refreshes at the rate of once every second. And as far as motoring across the lake I think it reads the speed accurately. However, if you're going very slow, single poling or spider rigging, it may show your speed ok enough but it won't show directional change fast enough. And this would gripe me to no end. In order to know if I was correctly heading toward a waypoint I had marked I would have to speed up too fast to get it to show change in direction and many times I would over shoot the waypoint by the time the unit would catch up to my change in direction. My newer HDS Gen II updates the gps positioning at 5 times a second, and is much better. But I bought the point one antenna which updates 10 times a second and always shows which way the front of the boat is pointing at all times which makes it very easy to navigate to waypoints and I absolutely love it. So, in my opinion, whatever unit you're thinking about, if GPS accuracy is important to you then I would check out the gps refresh rate.