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  1. #1
    Phil's Avatar
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    Garmin Nuvi Accuracy

    I recently purchased a Garmin Nuvi GPS and decided to use it to validate my max speed on my Ultra 260X. To make a speed run, I reset the max speed to zero, place the GPS in my glove box and then go for about a mile run. While doing the run I look at the digital rpms and make note of the most constant rpm value. At the end of the run, I pull out the GPS and check the speed. You may ask, "Where is Phil going with this thread?" ... OK, using this procedure, is the Garmin Nuvi dead-on balls accurate? I know when I am sitting still at idle, the GPS sometimes reads 1.8mph, 2.6mph and 3.8mph. I know this is wrong because I am not moving at all... but what about when you are going fast... could there be a 3-5mph max speed error? I called Garmin yesterday and the tech rep said, "If the GPS says 79.6mph, you are going 79.6mph plus or minus 2/10s. It can never be 5mph off!"

    Please relate your experiences?


  2. #2

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    I have the same GPS and get the same thing going out of a wake area. Its good to go bud. All of them do that.

  3. #3
    Fast & Loud meeper's Avatar
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    the issue is your GPS is setup for Land use, you need a handheld model for watercraft.

    i also have a NUVI and two handheld units.

  4. #4
    Phil's Avatar
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    Quote Originally Posted by meeper View Post
    the issue is your GPS is setup for Land use, you need a handheld model for watercraft.

    i also have a NUVI and two handheld units.
    Are you saying that the Garmin Nuvi does not work well on water? and if so.... why not?

  5. #5
    Fast & Loud meeper's Avatar
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    Quote Originally Posted by Phil View Post
    Are you saying that the Garmin Nuvi does not work well on water? and if so.... why not?
    Yes i am, it is for automotive use, i think there are settings for land or water use, not sure on your model though

  6. #6
    Phil's Avatar
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    Quote Originally Posted by meeper View Post
    Yes i am, it is for automotive use, i think there are settings for land or water use, not sure on your model though
    I'm sure there are functions that you would use in your car where they would not be applicable to an on-water scenario. But, isn't the speed functions of the GPS the same on water as it is on land?

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Phil View Post
    ...Garmin Nuvi GPS and decided to use it to validate my max speed on my Ultra 260X.

    To make a speed run, I reset the max speed to zero, place the GPS in my glove box and then go for about a mile run.

    ...At the end of the run, I pull out the GPS and check the speed...
    Put the GPS in a sealed plastic baggie, and strap it down where you can see it while you are riding.

    The GPS needs a clear view of the sky over a wide arc to provide maximum accuracy. And it needs strong reception of those satellite signals.

    Bouncing around in the glovebox, with your body partially blocking the satellite signals, can limit GPS position accuracy.

    With reduced position accuracy, the maximum GPS speed calculation can also be inaccurate. The average speed displayed may jump around, but will mostly be accurate.

    Watching the current speed number displayed will give you a better idea than relying on the GPS log of the peak speed displayed. That peak number might have been incorrect.

    You have no way of verifying whether the peak recorded speed matched the continuous top speed over many seconds, unless you watch the display as you ride. Some GPS have the ability to analyze a stored track afterwards, and show the speed over many small segments.

    Have a look at this;
    31.4 MPH.

  8. #8
    Hydrotoys's Avatar
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    Quote Originally Posted by K447 View Post
    Put the GPS in a sealed plastic baggie, and strap it down where you can see it while you are riding.

    The GPS needs a clear view of the sky over a wide arc to provide maximum accuracy. And it needs strong reception of those satellite signals.

    Bouncing around in the glovebox, with your body partially blocking the satellite signals, can limit GPS position accuracy.

    With reduced position accuracy, the maximum GPS speed calculation can also be inaccurate. The average speed displayed may jump around, but will mostly be accurate.

    Watching the current speed number displayed will give you a better idea that relying on the GPS log of the peak speed displayed. That peak number might have been incorrect.

    Have a look at this;
    31.4 MPH.
    EXACTLY what he says. You can't throw a gps in a glove box and hope it works right. Note my signature.

  9. #9
    Phil's Avatar
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    Quote Originally Posted by K447 View Post
    Put the GPS in a sealed plastic baggie, and strap it down where you can see it while you are riding.

    The GPS needs a clear view of the sky over a wide arc to provide maximum accuracy. And it needs strong reception of those satellite signals.

    Bouncing around in the glovebox, with your body partially blocking the satellite signals, can limit GPS position accuracy.

    With reduced position accuracy, the maximum GPS speed calculation can also be inaccurate. The average speed displayed may jump around, but will mostly be accurate.

    Watching the current speed number displayed will give you a better idea than relying on the GPS log of the peak speed displayed. That peak number might have been incorrect.

    You have no way of verifying whether the peak recorded speed matched the continuous top speed over many seconds, unless you watch the display as you ride. Some GPS have the ability to analyze a stored track afterwards, and show the speed over many small segments.

    Have a look at this;
    31.4 MPH.
    Now that makes a lot of sense. I guess the GPS could have been bouncing around in the glove box to get a 5mph spike. BTW, I love your 31.4MPH video.... but maybe you were really going 134mph and the GPS wrapped around!

  10. #10
    halfassjack's Avatar
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    there are many things affecting gps readings......current (water speed-flow) is one of them.

    David

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