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Thread: speed... GPS vs speedo

  1. #1
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    speed... GPS vs speedo

    right, heres one for ya... 3 different GPS gadgets in my near new work ute, all GPS read at 100kph 5k less... same in the car and on the bike... weird shit...
    OK so, whats more accurate...
    heres the thing, I was told car manufactures in NZ set speedos at 4kph under true reading... bull shit or not? I cant see it, as they need to be accurate... so why is GPS always reading slower... GPS NOT so accurate, or is it?
    Mr piggles nicks ya for speeding... is this GPS speed, radar speed, speedo speed, laser speed... what the fuck is accurate???

    ps: I expect the 3 GPS units (dash cam, phone and radar detector) work of the same satellite hence all the same reading?
    Whos in the know here?
    cheers DD
    (Definately Dodgy)



  2. #2
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    Gjeez dangerous, take some care in explaining what device reads what.

    Either way the GPS cant be wrong if you use it correctly.

    Long straight road, try 100kph and hold for several seconds.

    If GPS say 95 then you are doing 95.

    Vehicle speedos are optimistic.

    Both triumphs were 7% out. Van was 12% but back to commercial tyres and it went to 10%. Car is about 5% optimistic.
    Wife's Corolla is weirdly accurate.


    Google how GPS works and history, its actually interesting.

    But consider this. You're the US military, and you want to send a care package full of surprisingly hot treats to a baddie.
    Your platform is traveling at ninetity-billion mph (US measurements here sonny boy) and a 1% error will land it in another country. Not useful.
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  3. #3
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    The vehicle is not legal if it under-reads (you're doing 105 real, and it says 100), so often manufacturers, for safety, will deliberately make the vehicle over-read.

    Bear in mind, the difference in speed reading on new vs worn out tyres can be 1-2kph, tyre pressure can alter the accuracy by the same, or more. Change the tyres to something else and just about anything is possible.

    GPS readings will only really be accurate if you're on a straight road at a constant speed. The GPS is actually anticipating where you'll be in the next second or two based on the data, then corrects if you're not actually there, hence a lag in you slowing, and the GPS showing that.
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  4. #4
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    just did a calculation it appears on a 19inch front from new to legal worn there's a 1% difference in circumference for starters, car tires usually have a bit more tread so there's maybe 1.5% to start with variation. add to that manufacturers tollerance in speedos etc it's no wonder they mar em down, or risk lawsuits for getting tickets maybe. GPS all the way, I think if a satelittle is smart enough to stay in the sky it's smarter than your Toyota Rav 4

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    Dangerous, your logic is dangerous. Correct logic is the GPS is always reading lower because that is the correct speed. Your speedos will reader higher, not lower than true speed, and you'll be going slower than they say. That is the deliberate conservative error built into the speedos so you can't blame the manufacturer for being caught by the cops for speeding.

    So the manufacturer sets the speedo to show over the true reading not under as you stated. I'm guessing if you heard that story from someone else they meant to say you will be going under what the speedo says.
    Cheers

    Merv

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    My older Garmin in the trusty 04 Rav 4 both agree on speed readings , the Garmin in the 11 Kizashi says the Suzi is 4kms under at 100kph the roadside speed read outs agree with the Garmin and the fact other cars catch me up and the ones in front creep away while mine indicates 100kph.

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    Slow down Rossi.

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    Quote Originally Posted by dangerous View Post
    I was told car manufactures in NZ set speedos at 4kph under true reading... bull shit or not?
    Bull shit. Definitely. Almost all speedos read slightly high compared to GPS: my bike 6kph, my car 3kph. Akthough I should check the car again - new tyres.
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  9. #9
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    Yeah I fooked that up... Ment to say speedometers read 4ish above actual
    So if GPS is more the accurate then what does piggles do you with...
    By this I'm safe to sit at speedo 105kph and not get remed as the GPS says 100
    cheers DD
    (Definately Dodgy)



  10. #10
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    Quote Originally Posted by dangerous View Post
    Yeah I fooked that up... Ment to say speedometers read 4ish above actual
    So if GPS is more the accurate then what does piggles do you with...
    By this I'm safe to sit at speedo 105kph and not get remed as the GPS says 100
    If you are going at the correct speed according to GPS the piggles should have no reason to be doing anything with you, if by piggles you mean the cops.
    Cheers

    Merv

  11. #11
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    Quote Originally Posted by merv View Post
    If you are going at the correct speed according to GPS the piggles should have no reason to be doing anything with you, if by piggles you mean the cops.
    LOL... Yeah but that wouldn't be my luck, sitting on 95 drive buy points, so 103 0n the dash and 96 on the GPS, and I still crap myself when the detector goes off
    cheers DD
    (Definately Dodgy)



  12. #12
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    On my Triumphs I could correct error with TuneECU sw so when I look down I dont have to do math, I know if I need to hit picks if I see a collector. Newer ones dont run same SW sadly.
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  13. #13
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    Pretty much every bike I've ever owned has displayed the speed 5-10% higher than it actually is. Except for a KR150, that read like 40% out at 100 km/h. (It was great fun doing my full licence test on it and only having a rough idea of my actual speed.)

    If it puts the original poster's mind at rest, I've ridden past police many times doing 110-112 indicated. They've never cared in the slightest because their radars are measuring the actual speed, and 110 on those speedos was pretty much bang on 100.

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    I calibrated the speedo in my current car. You need cruise control (to keep the speed steady), a stopwatch and some measured kilometer strips (like the 5km posted odometer strips on the d'Auckland southern motorway approaching Bombay).

    At 100km/hr. a km takes 36 seconds. You set the car's speed with cruise control and time how long it takes to cover one km. Longer than 36 seconds, you are slower than 100 and less time means faster than 100. I found 105 on the car speedo matched the 36 seconds per km.

    I ran my GPS at the same time. It agreed with the stopwatch. 105, at 36 seconds per km, was exactly 100km/hr on the GPS.

    So you can believe your GPS. Most of the time...certainly under those conditions where you can maintain steady speed and a straight line for a decent distance.
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  15. #15
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    Firstly NEVER tell the cop about your GPS device if your disputing his claim. As all you are doing is announcing another treasure trove of evidence that he can now seize and use against you.
    Same for any of those phone apps that track your ride and overlay it on a sat map for social media bragging

    A truckie mate made the bad decision to dispute a roadside ticket because his GPS said 96 (which is still same ticket) vs 98. Cop got real pissed off and several days later seized GPS records and he was now facing over 300 seperate occasions of speeding.... plus location data didnÂ’t match logbook rest times...


    Went along to watch the case as I knew matey had couple avenues of possible escape....
    Early on the judge announced that NZ courts had already accepted GPS speed data as 100% accurate in previous cases so he was buffered there.
    The judge was real hesitant about location data though (even though its linked if you understand how it all works) as that has never been used in nz court before.

    In the end the cops made major stuff up by presenting the evidence in a hearsay manner and they didnÂ’t know where the servers were so defence was denied right to contest accurateness and integrity of the stored data/evidence.

    Whole case was dismissed, judge was really pissed off.

    In over 2 million trucking kms IÂ’ve always found most trucks accurate to about 2km my car and bikes usually overread about 4km.

    Also in my circles it seems radar detector owners get more tickets, sometimes old school situational awareness beats technogy which always has its failings.

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