thompsondockside.com

After making sure my old speedometer worked on my Seacoaster (1960), I recorded about 32 MPH or so, with four guys (maybe 160 each) and two tanks of fuel... the lake had a small chop, but was pretty smooth. I didn't check it a bunch of times in different directions, but figured I was doing OK. The boat is 15' 10", and I think I was using a 10x12 prop.. 75HP 1961 Johnson.

Later I was riding in another boat, slightly larger, with four guys and the same motor, and that speedo showed 37+... That speedo was a digital one run by a spinner in the water. Mine is driven by the compression of the air in the tube from the speedo arm mounted to the transom.

Since I think the scenery was flying by as much when I was in my boat as the other, I am wondering how accurate these old speedos are... The mechanism looked good, with no corrosion when I looked at it..

And John P.... you regularly mention that you are moving x mph via GPS... Does your GPS really have the resolution to pinpoint the speed that closely?

John Hart

John,

I have a speedo which looks to be part of the original 1963 guages added at the dealership--or could have been with the new motor in 1969--on our Super Lancer. We also have a GPS in the boat. The original speedo reads about two MPH SLOWER than the GPS speed. In our 1953 Super Deluxe we installed a vintage speedo last year. It also reads about one or two MPH SLOWER than a handheld GPS we carried on the same boat. We also have a 2003 Regal, the Faria modern speedo reads about two miles per hour FASTER than the handheld GPS. All three have a pitot (sp?)tube pick-up on the transom.

John Thompson

I would say the GPS is very close. All the major boat test reports record speeds with gps. My gps is very close to the auto speedometer.

Real killer of speed would be condition of bottom, and smoothness.