Thursday, June 06, 2013

Speaking Of Traffic.......

I drive a popular 1600cc hatchback, and along with millions of other people I have a satnav device. The satnav has a speed readout that shows the car's speedometer to over-read by 3 mph at 30, up to 6mph at 80. My previous car was the same.

A couple of points arise from this:- firstly, that if I set the cruise control at the limit in long camera-monitored roadworks, I am going about 4 mph faster than other drivers who are relying on their speedometer reading. Secondly, I am unimpressed by people who grumble about getting a ticket for doing, say, 36mph in a 30, because their speedometer almost certainly read nearer 40.

I know a lot of people who have done speed awareness courses (including several colleagues, a District Judge, legal advisers, and my own daughter) and all but the most cynical have admitted that the course is useful and thought-provoking.


  1. This "error" is designed in to all car speedometers, and has been for many years. There are two reasons they all over read. One is historic, in years gone by manufacturers deliberately made them over-read as an advertising ploy. When tighter Type Approval came in an official tolerance was adopted, of -0% to +10% +4km/h of the true speed or -0% +5% +10km/h.

    The reasons have to do with ensuring that speedometers never under-read, even when tyres are new or over-inflated (increasing wheel rolling diameter causes the indicated speed to be lower).

    My own car seems to indicate a consistent 10% over the true speed below 50mph, then a consistent 5mph above the true speed above 50mph. I've no idea why, but other owners of the same model have reported a similar pattern.

    1. I agree with Bystander regarding the benefit of speeed awareness courses versus fines. Financial penalties are resented and do nothing to make the driver drive any better.

  2. M'mm, but are you aware that insurance companies are now requesting details of any of these courses attended, and bumping premia accordingly? It may be cheaper to pay the speed fine and take the points.......

  3. An ex-NorthernJP6 June 2013 at 22:30

    Has anyone studied whether those attending speed awareness courses commit fewer further driving offences than those who are fined and given points on their licence?

  4. In these days of GPS, electronics and computers in cars, is it about time we had speedometers that actually tell us the speed we are travelling at regardless of tire size and inflation?

  5. I remember being in a car in Australia and hearing a really annoying bleep each time the speed limit was exceeded. It didn't stop the motorist speeding but nobody, in that type of car at least, could say they were not aware they had broken the speed limit.

  6. The satnav will also warn when the speed limit is exceeded. It can also warn of speed cameras, but woe betide you if you're caught using this facility in France. The resulting fine is a whopping 1500 euro!

  7. In France, mine warns me when we enter a 'dangerous road segment' and reminds me of the speed limit. Oddly enough, the authorities similarly recognise the danger and place a speed camera there.

  8. It's a bit like our sentencing powers, come to think of it: they usually over-read by about 150%.

  9. I have also carefully calibrated my Nissan’s speedometer, and found a 9.5% overestimation. Consequently I dial 76 into the cruise control on the motorway, and I do enjoy the slight oneupmanship of overtaking people as they brake at speed cameras. I contacted Nissan who replied that my observed error was “within design parameters”.

    Nevertheless, if it is overestimating the miles per hour by nearly 10%, is it also overestimating the miles per gallon and, indeed, the miles travelled by the same margin? Though the first may be excusable, I don’t think the last two are.

  10. Bear in mind that the car speedometer has to comply with legislation saying it cannot underestimate speed, but can underestimate it by upto 10%. I.e. it is set such that it reports the very top of the error bar on the measurement (in principle it should be near perfectly accurate to the rolling road it is tested on, but that calibration must accept a year's drift between MOTs and the error bar of the rolling road itself).

    GPS devices etc are presumably set to report their best determination of the value - from which the error bar will extend five-ten percent each way, maybe. So you could really be doing 33 when the GPS reads 30... especially, the accuracy of this measurement will depend on what satellites are in view in a given moment... many systems will keep reporting a speed even if their last measurement of it was a second or two ago.

    Caution should be used with the GPS device-reported speed, in any case.



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