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Thread: GPS Speedometer?

  1. #31
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    Quote Originally Posted by Shoryuken
    I have a delorme GPS and Street atlas 2k4, and I've found the speedometer in 2k4 to be a bit 'iffy'.

    From 0 - ~60 mph the GPS speedometer matches my car's almost exactly.

    From 60 - 70 mph, the GPS reads about 4 mph slower than my car speedometer
    From 70 - 80 mph, the gps reads about 7 mph slower than my car speedometer.
    From 80 - 90 mph the gps reads about 12 mph slower than my car speedometerr.

    I haven't been much over 90mph.

    At first, I thought it was my speedometer, but through extensive ghetto-testing (involving my wife following me and 2 cell phones) it appears that my car speedometer is correct ( +/- 1-2 mph).

    Anyone else have any problems? This is over a fairly long sample, with the cruise control set in the car, the gps speed is consistantly off at higher speeds.
    I've got Delorme SA 2005 and did a set of timed trials between mile markers with the car in cruise on flat ground set to 60 mph. This is a VW and the rumors about the speedo lying are TRUE!

    I did the trial because there was like a 4 mph difference between the speedo and the GPS output and I wanted to know which one was accurate.

    When the speedo would read 60, the GPS would read 56. I timed it and sure enough, 56 seconds or 56 mph.
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    I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
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  2. #32
    FLAC Jahntassa's Avatar
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    Most speedos are unreliable as you get higher in speed. After about 80 Mph, you should never trust your speedo to be telling you accurate speed, unless you've verified with a GPS.

    Whatever happened with the earthmate software? Was there a link and I missed it?

  3. #33
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    Exclamation For the record

    US Federal laws indicate that the speedo must be +/- 5% but there is little actual checking or enforcement due to different laws in the countries of manufacture.

    EU for example says the speed should always show higher speeds than actual speeds... never show slower, but based on the type of vehicle it can be read 110% + 4 to +8 Km/h... This only applies to speeds over 40km/h and no restriction applies below 40km/h...

    So higher speeds result in larger variance.

    Most other countries also have similar rules with similarly loose enforcement. The US has the strictest but allows both + & - variances. State by State also have other rules and California have 3% I think...

    GPS' accuracy depends upon how many satellites it uses as well as frequency but typically is much more dependable and accurate than a normal vehicle's speedo.

    Your transmission wear, tire wear and rear-end gear wear will all effect the accuracy of the speedo. Mechanical speedos have wear on gears and cables that also add to the trouble. Electronic sensor based speedos also have tolerance issues with the components they use. Typically the resistors, etc. used in the system will have tolerances from between 3% to 20% and will change over the life of the sensor... when was the last time anyone changed their speedo sensors, cables, gears, etc.?

    Old GPS may have been less accurate in location detection, but today's are much better. And imagine the scale... If the GPS is accurate to within a few meters, it will be able to read accurately as low as 1mph which is about 26.667 meters per minute or .444 meters per second... it will sense the movement even at that speed while most mechanical speedos won't sense the movement below about 5 to 10 mph depending upon the quality... 5 mph is almost 3 meters a second... more than enough for GPS to sense it.

    And how often any of us drive that slow... I would rather know my Speedo was accurate 187km/h because the police where I live won't chase if you are over 180km/h...

    Cheers,

  4. #34
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    Exclamation Also

    There are documented cases where the GPS has been used to fight speeding tickets based on police equipment which also has variances greater than the law...

  5. #35
    FLAC DodgeCummins's Avatar
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    If your speedo is wrong, then get it calibrated.

    The GPS has errors also...If you are going up or down a hill, it will not match your actual speed.

    It is accurate to 100' or so...so while you are standing still, it thinks you are moving.

    The faster you go, with a more constant airspeed and altitude (like in an airplane) the more accurate it will be.

  6. #36
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    Yes, the GPS has errors but unless you buy a poor quality GPS it will not be effected by hills, that is why they use multiple sats on the system. The triangulation between several sats will even detect altitude changes. Also used on Aircraft...

    As I mentioned Speedos are not accruate at all at low speeds, which is why most countries in the world don't try to dictate their accuracy... And if you are sitting still, well we usually know that... I have had several cars which wouldn't ready any movement (using a speedo) until I was at about 15 to 20 MPH... I have never seen a quality GPS that would be fooled as much...

    But I live in Japan, perhaps the GPS systems are more advanced since they are included with even cheap cars these days. The last one I tested even knew what side of the street I was on and was much more accurate than 100'.

    Cheers,
    By the by, I like your Dash pic DC... the Ram image looks absolutely evil... I love it...

  7. #37
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    Road Runner has a built in speedo, as does iGuidance GPS software.

    GPS is more accurate than speedos. I actually was involved in a class action lawsuit with honda, and won, because their cars were reporting more miles than actually driven, and it was a lease, so they had to pay me some cash back since my odometer was over - mileage when I turned it in.
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  8. #38
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    The 100' or so is likely a worst case scenario. I have an old and reasonably inexpensive Royaltek Sapphire GPS and the position wanders within about a 30 foot area when sitting still.

    It consistently shows my speedometer to be inaccurate by a few miles an hour at highway speeds. I've timed my car on one of those highway measure mile courses with the cruise control set on flat ground and found the GPS to be much closer than the speedo.
    Quote Originally Posted by ghettocruzer View Post
    I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
    Want to:
    -Find out about the new iBug iPad install?
    -Find out about carPC's in just 5 minutes? View the Car PC 101 video

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