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Author Topic:   Speedometer readings?
63WHALER posted 07-28-2003 04:52 PM ET (US)   Profile for 63WHALER  
I have an 1989 15'Striper with a 60hp Johnson and stingray foil and ss prop, not marked or polished. Ran at 5800-5900 RPM,s at WOT and registered 35 mph on speedometer. I know the speedometer is not the most accurate way to measure mph but was wondering if these stats. seemed about right? Generally speaking do the speedometers measure plus or minus or can't really tell?

I'll add myself @ 190# and the mrs. at Uh-Oh 160# , 12gal. fuel.

Thanks for any feedback!

Jamie 20 outrage posted 07-28-2003 09:07 PM ET (US)     Profile for Jamie 20 outrage  Send Email to Jamie 20 outrage     
Speedos are rarely accurate, borrow or buy a Garmin GPS and see what the reality is. Have fun!!
JohnJ80 posted 07-28-2003 10:51 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I just went through this exercise.

I have a pitot tube speedo, a fishfinder with paddle wheel and a GPS.

I found that there was pretty good agreement (+/- 10%) and it tracked up until about 35mph. From there, the FF and the speedo went wacky - tracking way, way high. At my top end per the GPS of 40.9mph, the speedo said 47mph and the FF said 51mph.

So, the error is not linear as a percentage - at least in my case.


jimh posted 07-29-2003 08:14 AM ET (US)     Profile for jimh  Send Email to jimh     
After some careful adjustment of the internal electronic calibration, the readings on my Lowrance SONAR from its paddlewheel speed transducer are in close agreement with the readings from my GPS with WAAS. Generally the difference is less than 1-MPH, and more often than not just a few tenths of a MPH. This is in the range of 2-30 MPH.

The accuracy of readings from a pitot tube speedometer is probably not as good at all speed ranges, as the dial calibration suggests by its non-uniform divisions. In the range of about 15-30 MPH the calibration appears to be best. The accuracy may depend on the quality of the device and will vary with the particular installation. If you get a high-quality speedo and you happen to have the same boat hull that was used to calibrate it--a ski boat probably--you may find the accuracy quite good.

jokor3 posted 07-30-2003 04:15 PM ET (US)     Profile for jokor3  Send Email to jokor3     
I spoke with a customer service rep. for Bottomline fishfinders today and was questioning him of the accuracy of the paddle wheel speedos on the 4300 fishfinder and he said they (4300) were accurate to within +/- 1-2 mph up to 60 mph. Is this statement as accurate as he states their fishfinders are?
Sal DiMercurio posted 07-30-2003 06:21 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
My Furuno paddle wheel is dead on with the gps & speedo, all three within 1 - 2 mph until 50 mph, then my speedo reads 3 - 6 mph slow.
If i'm running 60 it reads 55 - 57 while the sonar & gps are neck & neck.
Jerry Townsend posted 07-30-2003 07:59 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
63Whaler - generally speaking the speedometer will be accurate to within around +/- 5%. Some of the "cheaper" speedometers will be less accurate. But, one must also realize that the speedometer's speed indication is relative to the water. Therefore, in a river or with or against a tide, the indication from a speedometer will, by definition, be different than that from a GPS. ---- Jerry/Idaho

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