Speedometer 4MPH Off According To GPS

dtescher

Petty Officer 3rd Class
Joined
Jun 20, 2012
Messages
83
Mt speedometer is off by 4mph according to my GPS. Is there a way to test speedometer? It's original to the boat. 1997 Reinell 4.3GL PLKD #4110144649
 

matt167

Captain
Joined
Sep 27, 2012
Messages
3,630
Re: Speedometer 4MPH Off According To GPS

4 MPH is nothing. Supprised it's not reading 10 MPH or more off. Normal boat speedometers are not accurate and that is why the market is switching to GPS speedometers. The speedo you have is basically an air speed gauge that measures air pressure. The water pushes against the air in the tube and makes it work, but they are never 100% accurate
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Speedometer 4MPH Off According To GPS

The speedometer for your boat has nothing to do with your engine or drive. It's just a pressure pitot tube mounted on the transom. Very inaccurate.
 

Nickypoo

Petty Officer 1st Class
Joined
Mar 21, 2012
Messages
287
Re: Speedometer 4MPH Off According To GPS

Yes conventional boat speedometers measure stagnation pressure at the tip of a pitot. This isn't a very precise way to measure speed but it's simple and reliable.

GPS speedometers are more accurate, but only on land. What you are seeing on the GPS is your speed relative to land. You could be in a current where your water speed is different than the land speed. In this case the pitot may be more accurate.
 

Don S

Honorary Moderator Emeritus
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Aug 31, 2004
Messages
62,321
Re: Speedometer 4MPH Off According To GPS

You could be anchored in a 10 mph current with the engine off, and the speedometer on your boat would show 10 mph. But you are anchored and going nowhere. The GPS gives you speed over the ground, the speedometer gives you speed the water is passing the pitot tube.
 

dtescher

Petty Officer 3rd Class
Joined
Jun 20, 2012
Messages
83
Re: Speedometer 4MPH Off According To GPS

Good enough. I won't worry about the difference then. Thanks for the info.
 

82rude

Rear Admiral
Joined
May 8, 2012
Messages
4,082
Re: Speedometer 4MPH Off According To GPS

another name for them is lie o meter.
 

rallyart

Lieutenant Junior Grade
Joined
Jun 7, 2008
Messages
1,175
Re: Speedometer 4MPH Off According To GPS

I thought the Lie-O-Meter was the fuel gauge. :confused:
 

rallyart

Lieutenant Junior Grade
Joined
Jun 7, 2008
Messages
1,175
Re: Speedometer 4MPH Off According To GPS

Oh well, at least I know that Port means a red wine. ;)
 

etracer68

Ensign
Joined
Oct 11, 2009
Messages
906
Re: Speedometer 4MPH Off According To GPS

And not to be confussed with, RED, "RIGHT", RETURN.
 

cuenta

Petty Officer 2nd Class
Joined
Apr 26, 2011
Messages
137
Re: Speedometer 4MPH Off According To GPS

My brother made ​​a bigger hole to pilot tube connection
to trick the speedometer
he gain 10 MPH
jajajajaja..


Pitot_Tube-2.jpg
 

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
17,620
Re: Speedometer 4MPH Off According To GPS

FWIW - I have found that the pressure speedometers are most accurate in the mid speed range, like 30 - 40 mph.

it under states at lower speeds and over states at higher speeds versus the GPS. I often see wild claims of top speed for boats in Ad listings. . . I'm thinking they got those from the pressure speedo and not the GPS
 

Campylobacter

Chief Petty Officer
Joined
Aug 4, 2007
Messages
503
Re: Speedometer 4MPH Off According To GPS

I often see wild claims of top speed for boats in Ad listings. . . I'm thinking they got those from the pressure speedo and not the GPS

You are a far more charitable than I am, I assume they are lying. Or that all the 3.0's around here can do 50.
 

coolbri70

Lieutenant Commander
Joined
Oct 6, 2011
Messages
1,554
Re: Speedometer 4MPH Off According To GPS

when i'm checking my top speed with gps i like to run upriver then down, i figure the diffirence is the rivers current speed and my top speed in the middle, so even if your speedo is correct, it will be off from your gps speed if the water is moving too
 

Capt Ken

Commander
Joined
Jul 30, 2002
Messages
2,269
Re: Speedometer 4MPH Off According To GPS

I'm surprised the difference between running in hard water and soft water hasn't been discussed. We all know the boats run slower in hard water due to more hull resistance. Extensive studies in prop wash samples has proven this theory.
 

dan02gt

Chief Petty Officer
Joined
Aug 30, 2012
Messages
463
Re: Speedometer 4MPH Off According To GPS

FWIW - I have found that the pressure speedometers are most accurate in the mid speed range, like 30 - 40 mph.

it under states at lower speeds and over states at higher speeds versus the GPS. I often see wild claims of top speed for boats in Ad listings. . . I'm thinking they got those from the pressure speedo and not the GPS

That's exactly how mine is. It's within 1MPH of my GPS between 30-50, but by the time I get to the high 50s it's as much as 5 off.

I like to say the that the GPS slowed a lot of boats down.
 

agallant80

Commander
Joined
Oct 25, 2010
Messages
2,328
Re: Speedometer 4MPH Off According To GPS

Remember your GPS is going to show speed over ground, the meter in your boat will show speed over water, if there is a current it will add to your speed or take away depending on direction.
 

phonfish

Cadet
Joined
Aug 26, 2008
Messages
23
Re: Speedometer 4MPH Off According To GPS

Make sure you are not reading mph on one and knots on the other.
 

emilsr

Senior Chief Petty Officer
Joined
Dec 16, 2010
Messages
774
Re: Speedometer 4MPH Off According To GPS

Check your boat's speed on radar and you may find the GPS speed to actually be higher.

My first 2 boats didn't even have a speedo. Now I know why.
 
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