Speed limits speedometers vs GPS speed
Discussion
douglasb said:
The "10% plus" doesn't vary from region to region and nobody on here (or anywhere else as far as I am aware) has ever been able to show any paperwork for being charged at a speed of anything under limit + 10% +2mph (the starting point for charging) . Of course it happened to a friend's brother's aunt 17 years ago and she didn't keep the letter...
The 10% bit doesn't seem to vary but the 'plus' amount does... https://www.autoexpress.co.uk/car-news/106674/uk-s...There's also a chart at the bottom of that article showing the accuracy of a range of car speedo's at 30/50/60/70mph with Volvo (At least the XC40) seemingly the most accurate and the e-Niro being a consistent 3mph overstated.
J1990 said:
The 10% bit doesn't seem to vary but the 'plus' amount does... https://www.autoexpress.co.uk/car-news/106674/uk-s...
There's also a chart at the bottom of that article showing the accuracy of a range of car speedo's at 30/50/60/70mph with Volvo (At least the XC40) seemingly the most accurate and the e-Niro being a consistent 3mph overstated.
Yes. An article that still refers to ‘Association of Chief Police Officers’There's also a chart at the bottom of that article showing the accuracy of a range of car speedo's at 30/50/60/70mph with Volvo (At least the XC40) seemingly the most accurate and the e-Niro being a consistent 3mph overstated.
(Yes I know the NPCC took across the enforcement limits)
I don’t think the Scottish Police have adopted those thresholds.
This article (2021) shows a response (or not) from across U.K. Police Forces.
https://www.edinburghlive.co.uk/news/edinburgh-new...
douglasb said:
However as the GPS doesn't measure an instantaneous speed but calculates the speed based on the time to travel a measured distance in a measured time the speed is going to be an average.
Not true, I'm afraid. GPS speed measurement is based on dopplering the carrier signal from the satellites and produces an instantaneous measurement. If you have access to all the information produced by the GPS receiver (such as I get on my race car's data logger) then you get another channel which tells you the instantaneous accuracy of the speed measurement. It varies quite a lot.https://www.vboxautomotive.co.uk/index.php/en/how-...
Teem50 said:
Not true, I'm afraid. GPS speed measurement is based on dopplering the carrier signal from the satellites and produces an instantaneous measurement. If you have access to all the information produced by the GPS receiver (such as I get on my race car's data logger) then you get another channel which tells you the instantaneous accuracy of the speed measurement. It varies quite a lot.
https://www.vboxautomotive.co.uk/index.php/en/how-...
How come for a moment after I've stopped my GPS is still giving a speed reading > https://www.vboxautomotive.co.uk/index.php/en/how-...
Teem50 said:
Not true, I'm afraid. GPS speed measurement is based on dopplering the carrier signal from the satellites and produces an instantaneous measurement. If you have access to all the information produced by the GPS receiver (such as I get on my race car's data logger) then you get another channel which tells you the instantaneous accuracy of the speed measurement. It varies quite a lot.
https://www.vboxautomotive.co.uk/index.php/en/how-...
As your own link says, that's only the case for certain specific 'GPS Engines'.https://www.vboxautomotive.co.uk/index.php/en/how-...
It's something of a simplification anyway, because GPS doesn't actually transmit a 'carrier' and the signal's doppler is kind of dominated by the satellite doing something like 9000mph. Add in some signals bouncing off buildings in town, or losing the signal under a wet tree
The GPS in a phone or a Tom-Tom sat nav will have quite a lot of processing between the 'GPS engine' and the display.
One of the handheld GPS units I have for the boat is quite cunning, once it decides your speed is low and random, it assumes you've stopped and averages your position to make that more accurate. A 1mph error can be a very big deal on a sailing boat, or if you're hiking.
This is being massively over complicated!!
Easiest thing to do is next time you're on a nice straight flat road, compare GPS speed to your speedo for a few different speeds, then use that as your "calibration" for your own car.
Your speedo is a much better live readout than a phone GPS once you know how it behaves compared to true speed.
If anyone wants to spend a couple of grand having this done professionally with 100Hz IMU blended and RTK corrected GPS then feel free to PM me. The result will be the same.
Easiest thing to do is next time you're on a nice straight flat road, compare GPS speed to your speedo for a few different speeds, then use that as your "calibration" for your own car.
Your speedo is a much better live readout than a phone GPS once you know how it behaves compared to true speed.
If anyone wants to spend a couple of grand having this done professionally with 100Hz IMU blended and RTK corrected GPS then feel free to PM me. The result will be the same.
vroomtshh said:
I wouldn't expect so. It was 6 or 7 years ago, and wasn't planning on being interrogated on the internet about it 
I'm fairly confident it happened though. I even remember having to stop on the m74 on the way home from the speed awareness course to change a puncture. Proper ruined my day
Odd that the police forces up there say different I'm fairly confident it happened though. I even remember having to stop on the m74 on the way home from the speed awareness course to change a puncture. Proper ruined my day
blank said:
This is being massively over complicated!!
Easiest thing to do is next time you're on a nice straight flat road, compare GPS speed to your speedo for a few different speeds, then use that as your "calibration" for your own car.
Your speedo is a much better live readout than a phone GPS once you know how it behaves compared to true speed.
If anyone wants to spend a couple of grand having this done professionally with 100Hz IMU blended and RTK corrected GPS then feel free to PM me. The result will be the same.
Easiest thing to do is next time you're on a nice straight flat road, compare GPS speed to your speedo for a few different speeds, then use that as your "calibration" for your own car.
Your speedo is a much better live readout than a phone GPS once you know how it behaves compared to true speed.
If anyone wants to spend a couple of grand having this done professionally with 100Hz IMU blended and RTK corrected GPS then feel free to PM me. The result will be the same.
Craig_suke said:
Odd that the police forces up there say different
Of course you know everybody in the police forces up there, and they all say they have never icked anyone for doing sixty four in a sixty, so Vroomtsh must be making it up.Just because you haven't experienced a thing doesn't mean it's never happened.
Absence of evidence is not evidence of absence.
EU type approval determines that accuracy should be
https://eur-lex.europa.eu/LexUriServ/LexUriServ.do...
5.3.
The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h
It specifies tyre pressures of 2-bar and a temp of 25degC
My own observations having used several GPSs with several vehicles from different manufacturers is that the difference between speedo and GPS seems to be +3kph on newish cars using Cruise control on a fairly level road. I also have a 1990 BMW and that seems to massively over-read 10% and more, and by a larger %age the faster you go. I tend to rely on the tacho.
In France and Spain, the speed limit is enforced at +5kph, or +5% over 100kph, and so speedometer accuracy is pretty important. In the UK the tolerance is quite generous, by comparison.
https://eur-lex.europa.eu/LexUriServ/LexUriServ.do...
5.3.
The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h
It specifies tyre pressures of 2-bar and a temp of 25degC
My own observations having used several GPSs with several vehicles from different manufacturers is that the difference between speedo and GPS seems to be +3kph on newish cars using Cruise control on a fairly level road. I also have a 1990 BMW and that seems to massively over-read 10% and more, and by a larger %age the faster you go. I tend to rely on the tacho.
In France and Spain, the speed limit is enforced at +5kph, or +5% over 100kph, and so speedometer accuracy is pretty important. In the UK the tolerance is quite generous, by comparison.
My car speedo over reads by around 7-8% compared to GPS when travelling at a fixed speed on a straight road. I drive on motorways a lot especially on variable motorways with cameras, and others with long stretches of 50mph roadworks with average cameras.
I go by my GPS speedo, not the speedo in the car, and have done for years. I set the car limiter for a GPS indicated 41 in a 40, 52 in a 50, 72 in a 70, and so on, because as others have pointed out, no one has ever proved they have been ticketed for 1 or 2 mph over the limit (In England).
Travelling through long stretches of camera enforced 50mph roadworks my car speedo usually shows about 55-56mph, and I'm passing/overtaking most other vehicles at a reasonable pace, who are presumably keeping their car speedo at 49-50mph, but in reality they are doing about 45mph.
I go by my GPS speedo, not the speedo in the car, and have done for years. I set the car limiter for a GPS indicated 41 in a 40, 52 in a 50, 72 in a 70, and so on, because as others have pointed out, no one has ever proved they have been ticketed for 1 or 2 mph over the limit (In England).
Travelling through long stretches of camera enforced 50mph roadworks my car speedo usually shows about 55-56mph, and I'm passing/overtaking most other vehicles at a reasonable pace, who are presumably keeping their car speedo at 49-50mph, but in reality they are doing about 45mph.
Lord Marylebone said:
My car speedo over reads by around 7-8% compared to GPS when travelling at a fixed speed on a straight road. I drive on motorways a lot especially on variable motorways with cameras, and others with long stretches of 50mph roadworks with average cameras.
I go by my GPS speedo, not the speedo in the car, and have done for years. I set the car limiter for a GPS indicated 41 in a 40, 52 in a 50, 72 in a 70, and so on, because as others have pointed out, no one has ever proved they have been ticketed for 1 or 2 mph over the limit (In England).
Travelling through long stretches of camera enforced 50mph roadworks my car speedo usually shows about 55-56mph, and I'm passing/overtaking most other vehicles at a reasonable pace, who are presumably keeping their car speedo at 49-50mph, but in reality they are doing about 45mph.
Snap. I go by my GPS speedo, not the speedo in the car, and have done for years. I set the car limiter for a GPS indicated 41 in a 40, 52 in a 50, 72 in a 70, and so on, because as others have pointed out, no one has ever proved they have been ticketed for 1 or 2 mph over the limit (In England).
Travelling through long stretches of camera enforced 50mph roadworks my car speedo usually shows about 55-56mph, and I'm passing/overtaking most other vehicles at a reasonable pace, who are presumably keeping their car speedo at 49-50mph, but in reality they are doing about 45mph.
vroomtshh said:
If you read the original post, it didn't happen 'up here'. It wasn't an officer stopping me either. It was a mobile camera van. So I definitely assumed I was okay, until the hire company got in touch with me.
It makes no real difference to my life whether people on the internet believe it happened or not
How about proving it by posting a personal detail redacted copy of the 'ticket'?It makes no real difference to my life whether people on the internet believe it happened or not
In my experience, modern car speedos seem to be much more accurate, so much so that I wonder if they might use some kind of GPS 'correction'?
Both mine and the Wife's car read about 1mph under a GPS shown speed, both have built-in satnav so it wouldn't be that difficult for the manufacturer to combine mechanical readings with the received signal to give a more accurate reading - or maybe they're just better built than the olden days?
Unlike the speedo on my old Italian motorbike that is wildly optimistic - back in it's day it was claimed to be one of the fastest production bikes, around 130mph IIRC but, whilst it might show that on the speedo, in reality it'd probably struggle to 110...
Both mine and the Wife's car read about 1mph under a GPS shown speed, both have built-in satnav so it wouldn't be that difficult for the manufacturer to combine mechanical readings with the received signal to give a more accurate reading - or maybe they're just better built than the olden days?
Unlike the speedo on my old Italian motorbike that is wildly optimistic - back in it's day it was claimed to be one of the fastest production bikes, around 130mph IIRC but, whilst it might show that on the speedo, in reality it'd probably struggle to 110...
Super Sonic said:
Of course you know everybody in the police forces up there, and they all say they have never icked anyone for doing sixty four in a sixty, so Vroomtsh must be making it up.
Just because you haven't experienced a thing doesn't mean it's never happened.
Absence of evidence is not evidence of absence.
Cool story…Or looking at data provided by said police forces and their own guidelines prove he is probably talking rubbish. It’s all publicly available to see the guidelines….Just because you haven't experienced a thing doesn't mean it's never happened.
Absence of evidence is not evidence of absence.
vroomtshh said:
If you read the original post, it didn't happen 'up here'. It wasn't an officer stopping me either. It was a mobile camera van. So I definitely assumed I was okay, until the hire company got in touch with me.
It makes no real difference to my life whether people on the internet believe it happened or not
I did read your post, up there as in up north.It makes no real difference to my life whether people on the internet believe it happened or not
Prosecution Guidelines (those that are you know, like published publicly) for those forces differ on your story.
rdjohn said:
EU type approval determines that accuracy should be
https://eur-lex.europa.eu/LexUriServ/LexUriServ.do...
5.3.
The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
0 ? (V1 – V2) ? 0,1 V2 + 4 km/h
It specifies tyre pressures of 2-bar and a temp of 25degC
Not exactly.https://eur-lex.europa.eu/LexUriServ/LexUriServ.do...
5.3.
The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
0 ? (V1 – V2) ? 0,1 V2 + 4 km/h
It specifies tyre pressures of 2-bar and a temp of 25degC
It specifies tyre pressures 0.2 bar above the manufacturer's specified pressure (as it's a worse case to have tyres over inflated) and a temperature at the speedometer between 18 and 28 degC. Don't know why the temperature requirement is there but possibly due to old school mechanical speedos that would behave differently if very cold (or hot).
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