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Which speed is more accurate?

 
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redmax
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Joined: Jan 04, 2004
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PostPosted: Sat Jan 10, 2004 7:34 pm    Post subject: Which speed is more accurate? Reply with quote

I use TT2 bluetooth & iPaq 2210
Just a small observation....
The speed that is displayed on the ipaq is exactly 5mph less than my speedometer in my car. (Nissan X-Trail)
Which speed is more reliable/accurate the gps reading or my car?
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ttrevor
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PostPosted: Sat Jan 10, 2004 7:54 pm    Post subject: Reply with quote

Hi redmax

Ihave about the same reading as you have 5mph less than speedo,i do believe or i have heard that the gps is more accurate,but i stand to be corrected


trevor
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Dave
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PostPosted: Sat Jan 10, 2004 7:59 pm    Post subject: Reply with quote

GPS is more accurate providing you have a good satellite fix and that you are driving at a constant speed and on a constant flat surface, not travelling up or down a hill.
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redmax
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PostPosted: Sun Jan 11, 2004 9:24 am    Post subject: Reply with quote

Thanks
Nice to know there is a small margin for those moments of "reduced concentration" Laughing

Regards
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MILLWALLFC
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PostPosted: Mon Jan 12, 2004 1:05 am    Post subject: Reply with quote

I have read on here before that GPS is more accurate. However i believe it is not.

There is a school that i go pass everyday and it displays your speed as you go pass, a smile face if your under 30 and a sad face if your above 30. Every time i go pass this sign the speed on the speedometer is exactly the same as the sign.

However when using GPS it is always different as much as 5MPH.
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Dave
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PostPosted: Mon Jan 12, 2004 8:23 am    Post subject: Reply with quote

GPS generally is more accurate, but you've got to take other things into equation which make it inaccurate.

- Like is the school on a hill ?
- Are there tree's nearby or buildings that could be creating multipath errors ?
- How many sats do you have fixed ?

All of this can make a difference. On a flat, open road, and at a constant speed (and I don't mean using cruise control!), GPS providing you have a good reliable, strong fix, should be more accurate than the speedo.

Another thing you may find seeing that the speed check is outside a school, the local council/police are probably not going to show the true speed, speedo's are off by about 3-5 mph in most cars, and I suspect they'll calibrate it to what a speedo would usually show trying to make people drop their speed even more.
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Natashap
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PostPosted: Wed Jan 14, 2004 3:38 pm    Post subject: Reply with quote

Intersting discussion! I have just travelled some 10 iles with the cruise control set to 70 MPH. The GPS indicated a steady 63 MPH apart from a couple of times when it increased to 64-65 MPH presumably when less satelites were visible.
I then accelerated to 70MPH as indicated on the GPS and the car speedo was around 77-78MPH.
The question comes to mind as to the settings of speed cameras and then of course which system would the judge have to accept etc etc.
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Skippy
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PostPosted: Wed Jan 14, 2004 10:15 pm    Post subject: Reply with quote

Natashap wrote:
I then accelerated to 70MPH as indicated on the GPS and the car speedo was around 77-78MPH.


The manufacturers design speedometers to read between 1 to 10% fast. This allows for tyre wear, slightly different size tyres and a margin of error.

As for what the police will book you for, ACPO guidelines are to allow 10% of the posted speed limit plus 2 mph. This equals 79 MPH in a 70 area. In your case, where your speedo reads 10% fast, which means you probably won't be ticketed unless your speedo is indicating more than 87 MPH (79MPH true speed). This way, it's seen as a pretty fair cop as people who get booked are clearly exceeding the speed limit.

I have to stress that this leeway they allow is a guideline only and it is up to their discretion. The police may still choose to book you for speeding if you are doing 71MPH on a motorway in freezing fog or icy conditions. Police
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Dave
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PostPosted: Thu Jan 15, 2004 9:02 am    Post subject: Reply with quote

Skippy wrote:
The police may still choose to book you for speeding if you are doing 71MPH on a motorway in freezing fog or icy conditions. Police

And trust me in most cases they will clock you at just over the limit if it's icy or snowy, been there, done that, got the medal, £40 fine and 3 points.
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emjaiuk
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PostPosted: Thu Jan 15, 2004 1:41 pm    Post subject: Reply with quote

I've read this with interest.

Can any of the experts explain why the error seems to be 5mph at almost any speed, rather than a percentege of actual speed?

Under what circumstances could the speed shown on the GPS be BELOW 'true' speed?
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Natashap
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PostPosted: Thu Jan 15, 2004 6:05 pm    Post subject: Reply with quote

OK - but are safety cameras set at GPS speed or car speedo speed or even a more accurate system. How about police cars, they are supposed to be checked each day but to which criteria I wonder.
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Skippy
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PostPosted: Thu Jan 15, 2004 11:54 pm    Post subject: Reply with quote

Natashap wrote:
OK - but are safety cameras set at GPS speed or car speedo speed or even a more accurate system. How about police cars, they are supposed to be checked each day but to which criteria I wonder.


All speed detection equipement, including cameras must be carefully calibrated by the manufacturer and the calibration is frequently checked by the police. This is a legal requirement. I am told that the police normally do a check at the beginning and end of the shift and keep a note of the check in their notebooks.

Next time you are sitting in the back of a police car (traffic, not a panda car) take a look at the speedo. It is a specially calibrated one in 1 mph increments. The police use their speedo to cross check the accuracy of the LASER, RADAR, VASCAR and speed camera equipment.

As for the trigger speed of a camera, this depends on the politics of the particular area. I would guess that most cameras are set at about 20% above the speed limit. Speed cameras always measure "real" speed, so if the camera triggers at 85 MPH in a 70 area then the chances are that your speedo would be reading well over 90 MPH at the time.

emjaiuk wrote:
I've read this with interest.

Can any of the experts explain why the error seems to be 5mph at almost any speed, rather than a percentege of actual speed?

Under what circumstances could the speed shown on the GPS be BELOW 'true' speed?


The speed shown on your speedo depends on many factors. Your tyres (size, brand, inflation, wear), manufacturing tolerances in the speedo etc. I guess some manufacturers build the speedos to read a percentage fast, others a fixed 5 MPH fast. The only important thing to them is that it doesn't read too SLOW. It is interesting to note that while the speedo reads about 5% fast, the odometer in my car is very accurate (<1% over a 200 mile trip). 8O

GPS will report your speed as slower than it is under a quite a number of circumstances. They include poor reception, going up or down hill, going around a corner, sudden acceleration or braking. Basically it isn't accurate unless you are on flat ground traveling in a straight line. In this situation the accuracy is better than + - 1 MPH.
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DavidW
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PostPosted: Sun Jan 18, 2004 11:49 pm    Post subject: Reply with quote

So far as I can remember, the legal requirements in the UK (and probably the European Union) for a car speedometer is that it can display a speed up to 10% more than your true speed, but is not allowed to display a speed less than your true speed.

As a result, many speedometers show your speed to be a fair bit more than the true speed to ensure they comply with the legislation. As others have mentioned, there are various variables that mean manufacturers don't want to go too close to accurate (and risk displaying less than the true speed) - tyre pressures and vehicle loading are possibly chief amongst them, also tyre type and condition.

On modern electronic speedos, the systems are really very accurate indeed (as some have commented, the errors in the odometer, which runs from the same signal, are often very small) - but the speedometer is set deliberately to display a higher speed than true.

I keep meaning to check whether the average speed display on my Zafira's trip computer is more accurate than the speedo (choose an empty piece of motorway as level as possible, set the cruise control, reset the trip computer's average speed function, and compare the speedo reading with the trip computer's display after a mile or so). To my knowledge, the speedometer, odometer and trip computer all use the same electronic speed sensor on the drivetrain (as, for that matter, does the radio for speed dependent volume, and anything connected to the speed pin on the VDA 'carphone' connector).


If you change tyre or wheel size, as some people do, the chances are that the vehicle is illegal unless you also get the speedometer adjusted. There are some possible changes that finish up with the rolling radius so close that it makes basically no difference, but this is not always the case (particularly on some of the boy racer type cars you see).


The requirements for tachographs are different. I'm not sure of the precise specification, but it is a legal requirement in the UK that each tachograph installation is calibrated and sealed by an authorised centre. Driving a vehicle that has, by law, to carry a tachograph with an installation that is not calibrated and sealed is, outside a few special circumstances, an offence.


Police speed detection equipment has to be regularly calibrated and checked by the officers, as the previous poster explained. If information about the checks can't be produced then the information can't be relied upon. So far as their calibrated speedometers go, I believe they're set up on a rolling road periodically, particularly when the tyres are changed - as will happen fairly regularly on a traffic car as they cover phenomenal mileages. The officers check the equipment at the beginning and end of a shift by deliberately driving a measured distance at as constant a speed as possible and checking the time (I'm not sure whether the officers can use VASCAR for the cross check if they're carrying that system).


We had a fairly long thread about this in the past, which has now sadly expired from the forums (or if it hasn't, I can't find it). I think the safest advice is to figure out at what point on your speedo your GPS typically displays 70mph (which on most cars is probably 77-78mph) and drive at no more than about half the increment.



David
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ed_m
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PostPosted: Mon Jan 19, 2004 11:33 am    Post subject: Reply with quote

what davidw says about legislation is true to the best of my knowledge... and also the stuff about tolerances from vehicle load & tyres.

we use an downward looking optical sensor on our test vehicles at work and also see the speedo reading high by just over 5kph in the vehicle i'm thinking of.
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