Re: spinning wheel speedometer???
I haven't found that to be true, but granted my experience is based on one single experience with a paddlewheel. Between 4 boats between my fathers and mine, we have one paddlewheel speedo that came attached to the trandsucer of a fishfinder that was purchased about 15 years ago. From day one, that speedo has read about 15 to 20% too fast, regardless of experimenting with positioning of the wheel. All our other air pressure speedos are within 1 mph of the GPS, but need occasional cleaning and maintenance. However it's definitely true the paddlewheel will read at speeds lower than 10mph where the others won't, and it's never needed any maintenance or cleaning.
This thing hangs on the motor with a flexible aluminum arm, i've never seen one of these.