Accuracy of speedo

Discussion in 'General chat' started by Wynne71, Jan 16, 2020.

  1. Wynne71
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    Wynne71 Site Supporter

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    I’ve wondered how accurate the speedo is on the car for a while, so today I did some tracking with a GPS speed app on my phone.
    The car has factory 19” wheels with fully compliant tyres sizes, as per the book (they were fitted by Sytners before I bought the car).
    On a trip from Oxford to home (NE Bham) I tracked the car reading against the GPS tracker. I used cruise and my right foot to check the readings.

    Up to 40mph the two matched pretty accurately. 50 mph on the car showed 49mph on the app, 60 showed 58 and 70 showed 68. I didn’t take the car above 70, heavy rain and traffic, but I imagine the gap would increase as the speed rises.

    Maybe the GPS app isn’t spot on, but I’m more inclined to believe that than the car. They are generally calibrated to read under I believe.

    Interesting, well, I thought so.
     
  2. Nuclear
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    Nuclear

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    My speedo always reads2/3mph higher than my SatNav. Autocar & Motor did speed comparisons on all cars in their mag, 100% were out by some 1-5mph or so.
     
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  3. jwilalex
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    jwilalex

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    There’s actually an option in coding somewhere that allows speedo to show true gps speed or “correct” (I read that as ever so slightly lowered) speed. Mine has always showed on average about 3mph lower on the instrument speedo compared to GPS.

    I’ve got digital speed enabled in the area of the odometer reading (where you scroll round using the BC button on stalk) which is set to GPS reading so able to compare quite easily. I think most cars are dropped a couple mph which then caters for wheel size/speed anomalies etc. Also a possible reason why there’s a small fudge factor the cops allow on the speed guns; I forget the numbers as they differ across each constabulary but more info below:

    https://www.confused.com/on-the-road/driving-law/speed-camera-tolerances
     
  4. blackrat
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    blackrat

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    Speedos are more accurate than they have been in the past but will always OVER read so as to err on the side of caution and therefore in favour of the motorist ie at a recorded 40 you could be doing anything between 36 and 40. and so on...
    Obviously changing wheel/tyre sizes will effect the speed recorded by your speedo.
    GPS apps and satnavs will always be more accurate as the speed is taken from as the name says GPS but I wouldn't rely on it if you got caught speeding !!

    I believe I am correct in saying that speedos must be within 10% of the true speed and MUST NOT under read
     
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  5. Wynne71
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    Made me chuckle remembering the speedo on my 1987 Mini, it used to give a very approximate reading, 30mph was anywhere between 10-40mph. Bounced all over the place!
    I did understand that the slight over reading is there to protect the car makers from claims should the driver be inadvertently speeding.
     
  6. DiscusbT
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    DiscusbT Site Supporter

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    I would not rely on gps speed readout to be absolutely accurate. The ordinary system we all use is subject to many potential small errors. If you see a position plot of a stationary gps, over time it jumps about all over the place.
     
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  7. Peter
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    Peter WARLORD Site Supporter

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    Regulations for type approval are specific on how a speedometer must read, will always over read, as has already been stated. I've posted the specification and tolerances before, somewhere, I'll see if I can find them.

    The other factor to keep in mind, tyre tread depth (and pressure) also have to be allowed for in the calibration, to prevent under reading. New tyres will read a faster speed against GPS for a given speedo reading, than at the minimum tread depth.

    Peter
     
  8. Peter
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    Peter WARLORD Site Supporter

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    The Motor Vehicles (Approval) Regulations 1996

    Speedometers

    1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at all speeds mph. up the maximum design speed of the vehicle.

    2. For all true speeds up to the maximum design speed of the vehicle, the true speed shall not exceed the indicated speed.

    3. For all true speeds of between 25 mph and 70 mph (or the maximum design speed if lower), the difference between the indicated speed and the true speed shall not exceed—

    V / 10 + 6.25mph

    where V = the true speed of the vehicle in mph.
     

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