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Speedo vs GPS ?

Discussion in 'Off Topic' started by oldman, Jul 13, 2010.

  1. oldman

    oldman Super Moderator Staff Member

    Avalon Beach NSW
    G'day folks, I just did a run to Canberra & noticed a difference of about 3-4 kph between what my speedo read & what my GPS reckoned I was doing. The GPS showed slower than the speedo. I've heard of "built in safety margins", in modern cars & was wondering if anyone else had similar experiences, or knew which one would be the most accurate. Given the focus these days on mobile speed cameras, I'd hate to get pinged for running to the GPS if it wasn't reading accurately. :(
    No, I'm not a lead-foot, but I don't really want to hold traffic up either.....
    Cheers, Mark
  2. marcus

    marcus Member

    Moorebank, Sydney
    The question of accurate speedos gets asked a lot in newspapers' car sections and every article I have ever read suggests using a GPS to show your true speed. It states the cars speedo's tend to be up to 10% inaccurate.

    On the Easter long weekend I went to Noosa and trialled using my GPS only as a speedo and went past numerous Highway Patrol cars and cameras and never got booked. 110KM/H on my GPS was nearly 118-119 on my car so the theory to me seems true that GPS speed readings are more accurate. It now makes sense why when I'm sitting on 110 cars go flying past me but when I used the GPS i was then flying past them, but legally.

    Each car is different but I now know when I'm not using GPS (which is most of the time) in areas where the limit is 110 I can hover around 120 knowing I wont get booked.
  3. Deluxe

    Deluxe Active Member

    Similar to Marcus, I have found that my GPS records lower speeds than the speedo. I have used it in about 10 cars now with almost exactly the same result....therefore I question whether the GPS is accurate.

    To be on the safe side, I always run with the speed on the car.
  4. oldman

    oldman Super Moderator Staff Member

    Avalon Beach NSW
    Thanks guys, confirms what I had suspected. Cheers, Mark
  5. Van Housing

    Van Housing Well-Known Member

    Yarraville Vic
    Hmmm, I would personally favour the GPS (which is based on satellite readings of location) over the mechanical (and often inaccurate) speedo. To be on the safe side though, you probably should go with the *lower* of the two readings.
  6. Marlyn

    Marlyn Well-Known Member

    Eleebana / Newcastle
    My experience is similar my work car 09 Subaru reads 10% out, 04 Ford Explorer 100% accurate with Tom Tom, 73 Kombi very close to accurate, 83 Corona out , 92 Pulsar out
    considering I did 132,000 ks in the last 18 months ( for work alone)
    I stress about trusting the Tom Tom but clearly there is variability between cars if the GPS is correct
    cheers Mark
  7. Alpal

    Alpal Moderator Staff Member

    Melbourne Bend of Islands
    Having worked for VDO for 23 years (some time ago) be advised speedos are calibrated to avoid getting speeding fines. They are set to read from the actual speed to a percentage higher than the speed you are going.
    The Odometer is on the same cable drive so it will also read the same percentage out.
    Bigger or smaller outside diameter tyres will alter the reading. Smaller than standard tyres will increase the speed reading and larger tyres may get you booked for speeding!
    On a highway near you there will be a marked section of road with Km distances marked off. Travel at 100 and you should reach the next mark in 1 minute and the odo should clock over 1 Km.
    Bet you its not accurate over the 5 Km of marked road!
  8. Dubman

    Dubman Well-Known Member

    I sold GPS for years and there is always going to be a slight difference as the data travels to and from space..... in terms of distance a GPS is usually accurate within 5-10metres according to most manufactures so they may translate across to how accurate in Kph.
  9. Kombi Dad

    Kombi Dad Well-Known Member

    Bungendore, NSW
    Also have worked as an instrument fitter in the past. We fixed quite a few speedos and I can vouch for the quote. Also note that even under and over inflated tyres and wear on tyres will alter the readings slightly as the rolling diameter will change.

    Having used GPS for quite a while, especially in aircraft navigation, the speed reading will be quite accurate.


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