I have a B&G speed log which there are a number of ways to calibrate. Since there is always some current running the calibration requires a number of runs of a measured distance.
But if the bottom is fouled at all or the tranasducer is... which happens over time because of marine growth, the transducer performance changes. The GPS SOG gives an accurate reading of the speed, but not through the water and of course a computation of a course over time can give you an average speed. I have seen boat speeds of over 8 knots recently on a reach in flat seas and winds of 18 or so true. That is pretty hard to sustain for more than a few minutes, but I can hold the mid 7's in good conditions. With a fair current I have seen SOG as high as 9 and once in the race 11 for a few minutes. On ocean passages we can do 150 and have done 170. MY LWL is 29'. A clean bottom and prop makes a big difference and towing a dink slows you down as well.