STOP PRESS!

STOP PRESS! [/quote]

…

Who ever suggested re-calibrating a speedo with a GPS? Why would you want to?

No not exactly. That error rate is for position fixing.

The speed calculation is done by measuring the distance moved from position to position in a given time and then corrected using the doppler shift from the sat signals and is claimed by many manufacturers to be in the region of 0.1mph. It doesn’t get worse if you are doing 20 rather 80 also. So a percentage is not really correct.

For a second and then reads accurately. If it’s reading steadily then 99% it will be more accurate than your cars speedo.

Don’t think I can do a better job than the engineers at Trimble and Garmin. 🙂

If you’re really bored this is the explanation of the function that the vast majority of GPS receivers now have.

High Accuracy GPS speed

GPS based speedometer[/url]

Gobuchul, my car speedo reads perfectly accurately when the vehicle is stationary!

The point is you have no way of knowing how accurate your Garmin device is, and even less with a phone app (where both the app developer and the hardware provider can introduce error). Moreover because performance varies with signal and satellite position etc unless the device itself is configured to report the error you are blindly assuming that the number is right (and yet we seem to agree there is a short period of averaging or damping to make things more usable, and therefore must actually be wrong a lot of the time!)

I often see 10% difference claimed but I’ve tried it in a few cars and always see much less. I think it’s actually a bit of a dangerous Internet myth which added to things like the acpo guidance seems to mean people think it is “safe” * to drive at an indicated 73 in a 60. Now I am sure you can validate your speedo using the GPS if you are careful about it, but I remain sceptical that it is a good idea. if you want to run on the limit it might be worth bearing in mind that the speedo changes slightly with tyre wear.

* safe from prosecution not in terms of risk which is a whole different debate.

Is GPS speed 100% accurate 100% of the time? Obviously no.*

Is GPS speed considerably more accurate than a cars speedometer 99% of the time? Yes.

Where did I say that?

I am a little interested in GPS and use it at work in various guises a lot of the time.

Have a look at this if you are having trouble sleeping http://gpsworld.com/%5B/url%5D

*Unless it’s a RTK Differential GPS then it’s 99.9% accurate for 99.9% of the time.

I wonder if speedo accuracy is a part of the MOT test? Or is a case of if a speedo is present then is has to be functional.

ACPO policy used to be that 20s wouldn’t be enforced by police, which is why 20s tend to be full of traffic-calming measures. I guess from the Lancs example that that’s changing
The interpretation of the ACPO 10%+2 threshold for prosecution has also changed; it used to be over the threshold (36+ in a 30), but the threshold seems to be widely applied now (35+ in a 30)
I’m not sure how the 33 and 34 referred to above fit in and the other problem is that ACPO ceased to exist last year, and their policy was due for review in May 2015

Wouldn’t that theory mean they could potentially be 20% over?

Mine seems a fairly consistent 4mph over regardless of speed so not convinced the ‘safety margin’ is a percentage.

More accurate than GPS at that speed then – I’m sure that is handy in 0mph limits. More accurate than mine too, which reads -2mph when stationary, which is a infinite% error 😯

I agree with you about phone apps – not because the hardware is any worse, which it generally isn’t, but because you have an unknown app developer. Not the case for Garmin devices. You might not get a calibration certificate with one, but fundamentally they’re very accurate, far more accurate than car speedos which is the whole point (I have to admit this debate reminds me a bit of those arguing against DRS in cricket, where they say it needs to be 100% accurate before use, ignoring that without it the accuracy is far worse than that). I own and have owned numerous GPS devices – Garmin, Magellan, Suunto, Tomtom and have also used others including professional ones which do come with certificates – they’re all remarkably consistent with each other, crucially without the systematic error you get with a speedo. Sure you get some variation, but I’m certainly not suggesting relying on a GPS for instant speed – the long term accuracy is as gobuchul says.

I agree with you here – in fact I’ve already said as much, based both on checking my speedo with GPS and before that was available I checked the speedo in my previous car with a stopwatch and the odometer (odometer calibrated using motorway marker posts). Both methods agree with each other, and also confirmed by feedback from roadside automatic speed signs (which I presume do have some calibration). Error can be between 0% and 10% over, but IME it’s normally nearer 0%.

Personally I like to have all available info. Those who haven’t checked (note I’m not claiming calibration – I’ve done that for other things, and what I’ve done with my car speedo isn’t that) their speedo are probably the ones assuming 10% over-reading, something I know isn’t happening. Tyre wear makes a tiny difference to calibration, only about 2% over the full range of tyre wear, or in other words ~1/2mph at 30mph, which is less than the normal resolution of a speedo.

GPS may not be perfect, but over various phones with different chips and some using the Russian satellites, they’ve all read 5mph over my speedo in the same car. If not accurate, they’re consistent. Also the “your speed” signs at the side of the road tend to agree. In a 30 if I drive 35 I don’t trigger those signs, above 35 on the speedo and the sign flashes up that I’m over 30. Of course those signs may not be accurate 😉

As for caught by a gun or a static camera, a vast amount of the static cameras don’t really work (Surrey stretch of M25 cameras are now an exception 😉 ), though they might flash, just they aren’t set up to record. Historically due to lack of film, though with digital they might work but not sure many will have been upgraded. Cop at the speed awareness course basically said this is generally the case, but of course you never know which ones are active. He said the main bulk of fines will come from speed guns. Also mentioned that the average speed check cameras aren’t used for fines, purely for traffic control. Maybe just in that virtually no one speeds in an average speed check.

Other thing is if you’ve been flashed or seen a guy with the speed gun, wait 14 days for the letter. If nothing, then you’re in the clear as they must serve a NIP in 14 days. If you actually got stopped and get a talking to then you get the NIP or on the spot fine at that point.

By guns do you mean mobile cameras (given your mention of a guy with a gun, I’m not sure you do)? Years since I’ve seen a guy with a gun, I’d expect the number caught that way are tiny. I don’t have any evidence on fixed cameras, but understood they’d mostly been upgraded, and have certainly seen plenty of reports of them catching people.

I think all the last bit proves is that they tell lies at speed awareness courses. I’ve seen plenty of reports (including official ones) of people being done by those.

Yeah, mobile cameras I meant.

I’ve been flashed by a few static cameras with no result but was more than 5 years ago. Only time I’ve ever been caught is a mobile van. Sneakily hidden on a bridge just behind a gantry on the M4 in Wales coming up to the tolls. I forget what it was, 63 I think in a 50 so got offered the awareness course.

The M25 ones are still only a portion of them active. Mainly just the Surrey stretch I believe. They’ve never been active until recently.

🙂

Car speedo’s vary widely – my Mercedes seems to be almost exactly right (compared to the GPS) but all our other cards under-read by about 3mph or so.

I trust the speed the Satnav shows more than I trust the speedo.

Lots of speed awareness cameras in France. They tell you your speed and also suggest there might be a radar a bit further up the road.

My Fiat Ducato over read by at least 4km/h whatever the speed once the needle moved, on my Peugeot 605 the needle was just over real speed and my Current Dacia over reads by 3-4km/h at low speeds and then a fairly consistent 5%.