gps speed

G

greenstreak

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exMember
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Mar 8, 2004
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12942
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I just removed a Garmin 492 from the boat i purchased. When running the speed on the GPS matched the boat's speedometer speed. The new Garmin 3006c I just installed is 3 mph slower then my boats speed from the speedometer. I wonder how true the speed reading is from the GPS?

art
 
Art...GPS speed is SOG (speed over the ground) and the speedometer reads SOW (speed thru' the water). The difference is how much current is running and in which direction. They should only read the same when in still water.
ie. if you are doing 20mph on the speedometer and the current is astern at 5mph then your GPS should read 25mph, which is your true speed over the ground.
Conversely, if your speedo is indicating 20mph and the current is 5mph on the bow your GPS should be 15mph, for the same reason.
Another bit of interest is if you are tied up or anchored, and there is a 5mph bow current running, your speedo will indicate 5mph if it is that accurate, and the GPS will accurately read zero, because you are not moving.
David
 
Also look at speed averaging. Different units will average the speed over 1, 5, 10, or no seconds. Could make your boat look like it is going slower than it really is.
 
I would bet money that the GPS speed over ground is correct. I would not bet $0.02 that a pitot tube speedo was accurate ( also, as stated, pitot is speed through water ). Current could also be involved.
 
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