How accurate is the Speedometer?

[Deactivated] on 16/07/2014 - 19:35
Last edited 16/07/2014 - 19:35

Hey guys! I have an OBDII and I was testing the device out, it was so cool all the readings, turbo timers etc.
Until I got to the Speedometer, I was accelerating and decelerating and it occurred to me and my friend that the speed on my dash was significantly different on the dash as to the OBDII reading on the app. I even then went on the highway put the car on cruise, looking over to my Digital clock speed and my car reading was upto 5-6km/h under 100km/h on the (OBDII) app as opposed to my ongoing 100km/h reading on the dash.
Now I have heard of the cars being upto 2km/h less on cars for accuracy issues, I presume.
The car build date is July 2013, Holden Commodore VF, Color: Red (Red goes faster, or not? ;)

I recently ran into this artice;
http://www.drive.com.au/motor-news/how-accurate-is-your-spee…

How do speed cameras measure cars? Do they measure the speed at 98km/h or 100km/h to then to detect if a vehicle speeding or other wise doing 102km/h?
And why are new cars so inaccurate when it comes to Speed Tacho?
This all sounds confusing and such but I wanted to get an idea of what the community thinks about this Topic overall.

P.S. Some of you might say that my first problem is that its a Holden :p

Automotive

Comments

  • scrimshaw on 16/07/2014 - 19:46
    +1

    They use laser cameras and radars. For intersections they use a embedded detection loop embedded in the road.
    https://www.youtube.com/watch?v=Vz5pMBLHyMI

    According to the video in VIC (which may be outdated) there is tolerance level of 2km/h or 3km/h depending on whether it's a fixed or mobile camera.

    Unknown in other states.
    https://www.youtube.com/watch?feature=player_detailpage&v=sV…

    • [Deactivated] on 16/07/2014 - 21:03

      So for instance if my car was reading at 94km/h when the dash was reading 100km/h and then I was to choose to do 6km/h to cover the missing 6 from the OBDII in turn it would put me 106km/h on the dash does this mean that the laser cameras would detect me doing 100km/h? or 106km/h

      Sorry if this sounds confusing

      • windsurfer-sp on 16/07/2014 - 21:55
        +1

        Radars detect at your actual speed.
        Your speedo reads below your actual speed.
        A GPS/OBDII reads close to your actual speed (over and under marginally).

        So for my 2010 hatchback I set my speedo to the following speeds and drive through fixed radars happily in WA:
        Speedo: 63km/h, GPS: 60-62km/h, Real Speed: 61km/h
        Speedo: 105km/h, GPS: 100-103km/h, Real Speed: 101km/h

  • gevidian on 16/07/2014 - 20:03
    +13

    The accuracy of vehicle speedos is covered by Australian Design Rule 18. Until July 2006 this rule specified an accuracy of +/- 10 percent of the vehicle’s true speed when the vehicle was travelling above 40km/h.

    That is, at a true vehicle speed of 100km/h the speedo was allowed to indicate between 90km/h and 110km/h.

    An odometer accuracy of +/- 4 percent was also a requirement.

    From 1 July 2006 newly introduced models of a vehicle available on the market must comply with ADR 18/03. Also, from 1 July 2007 any newly manufactured vehicle (excluding mopeds) must comply with this rule.

    This new rule requires that the speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h. Significantly, this change means that speedos must always read 'safe', meaning that the vehicle's true speed must not be higher than the speed indicated by the speedo.

    That is, at a true vehicle speed of 100km/h the speedo must read between 100km/h and 114km/h. An alternative way to look at it is; at an indicated speed of 100km/h, the vehicle's true speed must be between 87.3 km/h and 100km/h.

    Significantly, this change means that speedos must always read ‘safe’, meaning that they are not permitted to read lower than the actual speed of the vehicle.

    Additionally, there is now no requirement to have an odometer, and therefore there is no accuracy requirement.

    This change was made to align Australian vehicle rules with those already in place in Europe.

    Note that some vehicle manufacturers chose to comply with the new rule before 1 July 2006. This is acceptable.

    • wharlie on 16/07/2014 - 20:37
      +4

      Good answer, well written and easy to understand.

      • scrimshaw on 16/07/2014 - 22:43
        +1

        Source: RacQ website
        http://www.racq.com.au/motoring/cars/car_advice/car_fact_she…

        • NosDan on 17/07/2014 - 16:48
          +1

          tl;dr

          An alternative way to look at it is; at an indicated speed of 100km/h, the vehicle's true speed must be between 87.3 km/h and 100km/h.

          in other words, dash says 100 cars really only doin 90, which is freaking annoying to me when im stuck behind you and your doing 10kph below the real speed limit… :P

          almost every car ive been in is right around the 5-10kph slower, this has been happening for ever (oldest car i personally know this to be the case was 1972, but probably before that too)

          where this changes tho is if you change wheels on the car. one of my current cars which had 14" wheels from the factory now has low profile 17's and the larger diameter tyres puts the speedo pretty much exactly correct (going by my fone GPS) so dash says im doin 100 im really doing 100.

          this is the android gps speedo app i use on my fone https://play.google.com/store/apps/details?id=de.meditgbr.an…
          if your an ifan, sucks to be you. get a real fone then use the above app ;)

        • Snoop on 22/07/2014 - 15:27

          @nosdan:
          Likewise, bigger tyres (larger OD) and mine is within 1-1.5kmh now!

          Ps. I have Digihud on the S4… But I also had an app on the iPos that was pretty good!

  • Davo1111 on 16/07/2014 - 21:18
    +2

    ADR stipulates speedos to be on the conservative side.
    Analogue speedos arent that accurate in the first place.

    If you're worried, use GPS or measure your speedo using distance/time

    • [Deactivated] on 16/07/2014 - 22:26

      I'm not really worried, more so curious :)
      So if my speedo is 6km/h off can i go about 5 km/h and it wont be as though i was speeding?

      • FutureTech on 17/07/2014 - 09:53
        +1

        You may, but the risk of accidentally getting over the speed limit and busted is significantly higher than just pretend your speedometer is accurate. Personally I wouldn't mind doing that(my speedo is 4km/h more than the actual) as the effect of travelling at 4km/h less is marginal (20min difference for a 8 hour trip from Sydney to Melbourne). However on the road (even somewhere with 60km/h limit) people find out I'm travelling marginally slower than the speed limit, get annoyed, go over the speed limit to quickly overtake me and then travel marginally faster in front of me. This happens all the time. Think about the risk and benefit involved and make your decision wisely.

  • Grent on 16/07/2014 - 22:29
    +1

    "An alternative way to look at it is; at an indicated speed of 100km/h, the vehicle's true speed must be between 87.3 km/h and 100km/h."

    And after the 'wipe off 5' campaign people sit smugly in the right lane of the highway indicating 95km/h but actually doing 85km/h in the 100 zone, making dirty faces at those that actually want to drive at or near the speed limit.

    • FutureTech on 17/07/2014 - 09:57
      +2

      They are wrong for staying in the right lane for longer than necessary, not for travelling at 85km/h in a 100 zone.

      • NosDan on 17/07/2014 - 16:51
        +1

        and they have the nerve to give me dirty looks when im sitting 10cm off there back bumper tailgating like a boss….. GET THE F#$K IN THE LEFT LANE IF YOU WANT TO GO SLOW MORON!!!!!!!!

      • Grent on 17/07/2014 - 23:38

        They are wrong on both accounts unless its raining that heavily that visibility is reduced or some other legitimate reason other than ignorance of the speed actually being travelled. Ignorance won't get you out of a speeding fine, nor should it get you out of a fine for impeding the flow of traffic.

        The highways are safe to drive a 1979 Kingswood on at 100km/h, so there is no reason someone in any modern car should be doing 85km/h due safety concerns.

        The argument of 20mins on an 8hr journey is weak too, because every time one gets caught at another set of lights by a couple of seconds due to a dawdling driver (and this seems to happen more than once every time I drive to work) there is another 5 minutes to add to the journey.

        • NosDan on 17/07/2014 - 23:44
          +1

          Grent 3 min ago
          <snip>

          looool… you still believe that speeding fines are anything more than revenue raising? do you really think it has ANYTHING to do with safety?

          cute…..

        • Grent on 19/07/2014 - 08:03

          @nosdan:
          What the?
          Where did I mention speeding or tailgating or anything other than driving at the speed limit?

  • scrimshaw on 16/07/2014 - 22:41
    +1

    In short, you do not have to worry if your speedometer is over-reading your vehicle's speed.

    In layman terms:

    A speedo over-reads if it displays 100kmh when the vehicle's actual speed is 90kmh.
A speedo under-reads if it displays 100kmh when the vehicle's actual speed is 110kmh.

    If the speedo is under-reading, then you are at risk of getting caught speeding since you are told by the speedometer you aren't speeding when you in fact are.

    Several things can cause speedometers to be inaccurate, ranging from calibration issues, diff ratios, vehicle load, and changes in tire pressure (or using non-standard tire sizes).

    GPS's tend to be fairly accurate in terms of speed measurement. So an easy way to 'calibrate' would be to take a GPS, drive down a long straight road at 100K's an hour and look at the difference in speed reported by the GPS unit and the odo.

    • Geewhizz on 17/07/2014 - 08:18

      GPS's tend to be fairly accurate in terms of speed measurement.

      I'd tend to argue that point somewhat, and it should be borne in mind that the readings from a GPS are not a valid defence against a speeding ticket.

      Regardless of whether a car's speedo is out, it is always going to be constant, yet I have found that the speed on the GPS can vary by about as much as 7 or 8 kmh with the same speed registering on the speedo (highway driving.)

  • boccos on 17/07/2014 - 09:23

    Hyundai has tested their cars and claims that their current vehicles speedo is more accurate than the GPS speed. Their drivers are advised to favour the cars reading rather than following the GPS reading.

    • Davo1111 on 18/07/2014 - 00:43
      +2

      I'm be interested to see how they came to that conclusion

      • NosDan on 18/07/2014 - 00:47

        their current vehicles speedo is more accurate than the GPS speed

        link to sauce?

        sniff sniff? is that bulls$%# i smell? :|

  • Davros on 17/07/2014 - 23:50

    GPS matches my speedo pretty much bang on.
    Ex Detective Commodore with 2 klm increments on the speedo.

  • Azn310 on 18/07/2014 - 00:36

    I think it also depends on your speed. In my case, if I drive at 60km/h the speedo would be 6km/h off in comparison to GPS. If I drive at 110km/h, then its 10km/h off.

    • Snoop on 22/07/2014 - 15:24

      Looking at your numbers it almost looks like there's some sort of constant variaton in the speeds you've recorded… approx 10%!

  • Euphemistic on 18/07/2014 - 11:44

    The speedo accuracy also changes as your tyres wear or in relation to tyre inflation. I've had GPS in cars for years and trust that it's more accurate than the needle on the dial. All of the cars I have had over read, that is the dial shows 100, the GPS showed anywhere between about 92-97 depending on th car.

    The reason they are acceptable if they over read is so that you can't blame the manufacturer if to you get a speeding ticket.

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