The picture above was taken about 4 years ago when the tachometer worked. Boat was running in cold weather with the wind behind me, with portable gas tanks on the rear seat (bow tank empty), using the 13 3/4" diameter x 23" pitch SRX over the hub prop. The true GPS speed was about 56+ MPH. Present tachometer will swing loose to higher RPM's.
The tachometer model is called the Red International Series made by Teleflex and is the stock gauge for that year of 1987 Glastron Carlson CVX-16.
I have a spare matching tachometer that is 0 to 7,000 RPM, but would prefer 0 to 6,000 RPM.