GPS speed gauge -vs- car speed gauge?

Discussion in 'General TomTom Discussion' started by SimmZ, Mar 14, 2008.

  1. SimmZ

    SimmZ

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    I got a funny question! Yesterday I do compare the speed indicated on my TT-One and the speed indicated on my car's speed gauge and there's a slight difference. When my car's gauge indicate 120 KM/h, the TT-One indicate around 117 KM/h. I think it's possible than my car's speed gauge was not accurate because I use smaller winter tires than original size (narrow tires give better grip on snow).

    Anyway, I'm just wondering if the speed indicated on the TT-One (or any GPS) was accurate or not really?
     
    SimmZ, Mar 14, 2008
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  2. SimmZ

    Smokiewolf

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    I would trust the GPS before your speedometer, unless it's been calibrated. Non-OEM size tires is the most common cause for the speedometer to be off.
     
    Smokiewolf, Mar 14, 2008
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  3. SimmZ

    Rick F.

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    Rick F., Mar 14, 2008
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  4. SimmZ

    Genius of Love

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    I would trust the GPS over your own speedometer. My car is very accurate, but my wife's reads 4 mph faster than the GPS indicates.

    On another front, as I am doing my research, the majority of motorcycle and scooter speedometers read 10% over their actual GPS speed.
     
    Genius of Love, Mar 14, 2008
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  5. SimmZ

    feejo

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    Hum... I have a XLS, and my speed at 120km show 100km on GPS I am suspecious. I tryed on ceveral cars and the car alway's goes faster than the gps.:confused:
    That tread is LOCK. I would say to mesure 1km on a road then run on it at 100km, it shoud take 10 sec.
     
    Last edited: Mar 25, 2008
    feejo, Mar 24, 2008
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  6. SimmZ

    boren

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    Calculation of speed by a GPS is very simple. GPS accuracy tends to drift over minutes/hours, but sequential calculations tend to give very close values. As such, the calculation of speed every 1s or every 250ms is very easy and it gives you accurate results.

    Car speedometers/odometers are normally terrible. There's a million things that cause them to read differently. I would trust the gps.
     
    boren, Mar 25, 2008
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  7. SimmZ

    feejo

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    Oh then my car is really slow then, that is why I don't catch much speed ticket.
     
    feejo, Mar 25, 2008
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  8. SimmZ

    dhn Moderator

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    No, in an earlier post, you said the GPS says 100 when the speedomter says 120. You are really going 100. Therefore, no ticket.
     
    dhn, Mar 25, 2008
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  9. SimmZ

    swb

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    I have one car where the speedometer is slightly lower than the GPS (so I have to be careful), and another car where it is about 5% over. It's all about the speedometer's assumptions on tire radius. Also, some years ago I had a car where the previous owner had replaced a gear in the feed to the speedometer, so the speedometer read 7% slower than reality. I got a ticket.
     
    swb, Mar 29, 2008
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  10. SimmZ

    equlizer

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    I trust my GPS over my car. Ive even tested it with some help from a police officer friend :)
     
    equlizer, Mar 29, 2008
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