Speedo or sat nav??????

Discussion in 'Tech/Mech General' started by gordoncbrown, Feb 12, 2012.

  1. gordoncbrown

    gordoncbrown Funster

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    Probably been asked before but here goes. Can any of the technical people tell me which is more accurate. My sat nav is always about 3 to4 mph out compared to my speedo. Not really an issue as I drive to the speed limit but curious. Also i did see a camera double flash today. It was for the other side of the road. I wasn't on the lines that are marked out and I was doing 55 in a 60. The car ahead of me was shifting though. Could he have set the camera off. No one else near the lines at the time either. :thumb:

    Thanks
     
  2. pappajohn

    pappajohn Funster Life Member

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    providing you are 'locked on' to enough satellites, then i would rely on GPS rather than a mechanical/electronic speedo any day.
     
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  3. steveclecy

    steveclecy Read Only Funster

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    Sat Nav all the time. They have to be accurate to work. Speedometers only have to be accurate to within 10%.

    Steve
     
  4. schojac

    schojac Read Only Funster

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    Speedos are always below true speed for legal reasons. usually about 10%
     
  5. GJH

    GJH Funster Life Member

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    I had a TomTom sat nav and changed to Garmin. They both show different speeds from the speedo but also differ from each other.

    So which is accurate? Any or none?
     
  6. Mr Colpot

    Mr Colpot Funster

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    As said - Speedo is set to read slightly faster than the actual speed. Sat navs tend to be more accurate - but be carefull if you rely on sat nav to drive by (speedwise) as if it loses contact with the satellites the speed will freeze until it makes contact again.
     
  7. MikeandCarolyn

    MikeandCarolyn Read Only Funster

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    My Garmin seems to be consistantly 3-4 mph slower than my speedo-in car and in M/Home.
    Think I'll go with my speedo and be really safe :BigGrin:
    Mike
     
  8. sabconsulting

    sabconsulting Read Only Funster

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    All the cars I have own have read faster on their speedos than actual speed, including my Honda which has a digital speedo.

    Remember that the accuracy of the speed reading will vary with variations in tyre diameter caused by different tyres, different tyre pressures and tread depths, not forgetting the inaccuracy of an analogue mechanical speedo dial.

    I suspect all manufacturers set them to read fast to avoid any legal ramifications resulting from an accident or prosecution where the car was travelling faster than the driver thought from the instruments.

    I wonder if GPS manufacturers who primarily make GPS for cars (e.g. Tom Tom) would include the same deliberate inaccuracy. I've always had Garmin units (3 hand-held + 1 in-car), but Garmin came from hand-held / marine navigation devices which they still make a lot of, so maybe accuracy of speed indication is seen as more important.

    The specification for my handheld GPS gives its speed accuracy to 0.05 m/s (0.11 mph). My Garmin Nuvi in-car unit seems to show the same speeds, so I assume it is similar accuracy.

    Steve.
     
  9. Reallyretired

    Reallyretired Funster

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    My two satnavs in the van (TomTom andSnooper) agree with each other and show a lower speed than the speedo. I work on the basis that the are the more accurate though I don't speed anyway.
     
  10. kickstart

    kickstart Funster

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    I always rely on my Garmin as I have a km speedo :Laughing:
     
  11. gozomike

    gozomike Funster Life Member Life Member

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    Hi.



    A few thoughts.


    In most cases the SatNav’s will be the most accurate but as been said the loss of signal would affect the accuracy. I also wonder if on bends the accuracy could be affected. This would depend on the sampling rate where the satnav would assume you had travelled in a straight line although you had travelled further. I am sure I have seen errors when taking some bends. My satnav certainly seems accurate when compared with the roadside speed indicators which are now quite popular in some French towns.


    As for speedo’s with a digital display whilst they appear more accurate I am not convinced as they still rely on the same sampling system as an analogue speedo where the tyre pressure and wear will affect the wheel diameter and thus accuracy.


    Mike
     
  12. rainbow chasers

    rainbow chasers Read Only Funster

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    Speedo will be more reliable than sat nav. I won't bore you with the technical geometrics and so on, as there are alot of forces in play - so I will put it simply.

    At sat nav is a guide, it has no purpose of being accurate. So when it finds two or three mini car sized satellites some several thousand miles away, it can tell you roughly where you are - but the slightest anomaly can throw them out. This can be good weather, bad weather or just the earths curvature:time ratio.

    A tracking device DOES need to be accurate, and they are within 1-3 metres.....makes quite a difference when you are covering 46 feet per second. It would make sat nav devices more accurate than trackers - and that is just silly.

    Satellites do topography, happily when stationary but find it difficult to measure accurately within a mm, especially a moving target. I don't want to baffle, so I won't get too techy with it.

    Accurate speed depends on calibration - when you leave the factory, you are within 10 percent or less - as vehicles age this becomes less accurate, you won't know unless you calibrate over a measured mile. You also have tyre wear, size etc which varies on different vehicles.

    So rely on your speedo as being more accurate - it has a better chance of being correct.
     
  13. schojac

    schojac Read Only Funster

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    Define accurate?

    if we are talking keeping the right side of the law then I agree.

    Being pedantic then no.
     
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