MPH: Speedo vs GPS

[boater2065]

Hello all,

I've seen this topic thrown around a bit but have never heard any definitive answer. I typically cruise my 2002 260 DA around 3500-3700 RPM. In calm conditions and little to no current, the built in speedo reads about 28-30mph while the GPS unit reads 20-24mph. To me the speed of ~28 mph sounds more accurate for that RPM, than the ~20mph that the GPS reads.

What do you all trust most for accurate speed, speedo or GPS?

And also, from people with lots of experience with this boat, what MPH SHOULD the boat be running, at that RPM?

 

[Creekwood]

GPS is accurate. Speedos are not.

 

[Lazy Daze]

Hands down, GPS is more accurate. Although "typically" a properly function pitot/speedo system is within about 2mph. Something to keep in mind is that GPS measures speed over ground, which is different than a pitot system.

I don't know what my 260DA went at those specific RPM's - plus there's way too may variables that come into play that can affect that. However, as far as top speed goes (GPS) I could get a solid 40MPH with just me and a "normal" load... typical gear and a good half tank of gas, or so.

 

[andy k]

I am not familiar with the GPS that you have. But just confirm that it is reading in MPH not KNOTS. If I recall correctly when I had my 260 It ran in the mid to upper 20's at 3600RPM loaded heavy. I had the 5.7 250HP carb with the Bravo II drive.

 

[bobeast]

GPS don't care nuthin, bout no wind or current. It just knows how far / how fast you've traveled on the planet relative to the satellites. Your pitot, on the other hand, is impacted by current, water density, hull angle and interference from other protrusions on your hull. Not to mention the manufacturers' "calibration" to compensate for it all.

 

[boater2065]

Thanks all for the responses here. I am indeed a moron and the GPS had been reading in knots, not MPH. It now all makes much more sense

 

[The Bill Collector]

Hi Jeff, your boating in my waters and the tide is either Ebb or Flood we have very little slack... The current around the Narrows can run easily run 4-6 knots. Also, are both units set for MPH or KTS because if they are not a knot is = to 1.1 mph. I have Boat speed and Speed over ground and rarely are they the same.

 

[sbw1]

Sometime when you are running from waypoint to waypoint for an extended period of time, for example more than an hour, write down your ETA from the GPS. This estimate is a simple time/distance math problem. In my experience that number is almost always right on the money. Therefore, the speed must be right on the money. Many modern boats no longer have speedometers, only GPS. BTW, you should be doing this already if you are operating in big waters.