Speedometers on boats are used for two purposes:
1) It's a great way to see what you can get stuck in the pitot tube, either engine mounted or transom mounted. When it stops working, you get to check the pitot opening to find out what clogged it, and then you can feel good about fixing something on your boat.
2) It's quality entertainment for your fishing partner. They get to see how really "fast" your boat is, although you know for a fact it's much slower than the speed being displayed. :lol:
Every once in a while, you get one that is fairly accurate. My current one has been close for the most part, and I have to say it's much more accurate than most I've had on other boats. Sometimes it takes a vacation and is a good 6 to 7 mph off of GPS speed, but for the most part, it's less than 2 mph off, which I thought was pretty impressive for a known problem gauge.
GPS speed is the way to go. Now if I could only figure out an accurate way to judge how much gas is in my boat (another gauge that is notorious for lying and causing headaches - although it is at least adjustable to a point.
All the best,
Glenn