SCLakes.com

Just out of curiosity, can anybody tell me how the speedometer on a boat reads?

I believe on boat motors there is a small hole in the foot that the water passes through and that registers the speed. I've never been in a boat that the speedometer was right. Most boats are usually 5 to 10 mph off. GPS is the best way to measure your speed.

Correct, most outboards run off water pressure from the front of the foot. Most that I've had read on the high side. I've seen some older speedos that had some sort of paddle wheel device that attached to the hull and could be calibrated, but I think they were mostly on competitive ski boats where maintaining accurate constant speed was critical.