Speed limits speedometers vs GPS speed

Speed limits speedometers vs GPS speed

Author
Discussion

TB404

Original Poster:

110 posts

165 months

Tuesday
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Hello people,
I want to confirm (or otherwise) my understanding of the subject.

So with all the bloody cameras everywhere I drive with Waze all the tie which is fine. I notice that on several different cars Waze reports about 10% lower GPS speed than the car's speedometer, I understand that's intentional. is this correct? the GPS speed shown in Waze is the actual speed that a speed camera would use to clock me?

On top of that we have the 10%+1 rule for speeding, which means one can drive 23mph in a 20mph zone )for example) and you won't get a ticket until you got to 24mph 'real speed' - i.e. GPS.

is this correct? am I missing something?

Thank you,


Scrump

20,192 posts

145 months

Tuesday
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You are correct, but best to leave yourself some margin for error and not rely upon gps speed with 10% +

J1990

592 posts

40 months

Tuesday
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The 10%+x rule varies from region to region, I wouldn’t rely on this as a measure for what speed you decide to trundle through speed cameras at though.

Speedometers are fairly accurate nowadays, in my last few cars I haven’t seen more than a 2-3mph variance from the GPS readout.


covboy

2,539 posts

161 months

Tuesday
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Car speedos cannot underead. Hence a usual discrepancy with a GPS indicated speed. Downside of relying on the GPS indication is its not exactly instantaneous

jondude

2,278 posts

204 months

Tuesday
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covboy said:
Car speedos cannot underead. Hence a usual discrepancy with a GPS indicated speed. Downside of relying on the GPS indication is its not exactly instantaneous
This is a good point, can remember spotting a police car late so put the brakes on - the dial on the speedo dropped like a stone but the GPS took a few seconds to work out the new speed. I think the GPS readout is very accurate so long as you are moving steadily along.

When my speedo reads 50mph, the Garmin says 47mph. I go with Garmin.

OutInTheShed

4,338 posts

13 months

Tuesday
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GPS can under read, because it has errors on every position it calculates.
GPS receivers bolted to buildings frequently indicate a couple of mph
Classically, it can under -calculate your speed around a curve by taking the straight line distance between two fixes.

Also bear in mind that the 'tolerance' on the speed the police or others observe may be used up by however they measure your speed.
So your true speed = 22, they measure that as 24, 'you're nicked.

Your GPS might say '22mph' but it's rounding down 22.49 down to 22.
It will be averaging over a short time.
The plod might e averaging over a different short time.

If you're driving to the GPS readout, they will nick you the moment you look away from it and drift up a couple of mph.

And that's without them 'trying' to get a nickable reading.

When you get old, you realise it's just easier to give yourself a margin.

dundarach

4,248 posts

215 months

Tuesday
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Because I now drive a Cashcow, I just stick at the speedo reading, happy in the knowledge I'll be slightly under.

Stick cruise on and relax.


douglasb

272 posts

209 months

Tuesday
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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...

GPS receivers aren't bolted to buildings. As GPS stands for Gobal Positioning by Satellite, there's a clue in the name of where the signals come from. The receiver in the vehicle needs to receive signals from (I think) 3 satellites to get a "fix". The more satellites it can "see" the more accurate the indicated speed will be. 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. Of course, if you are doing a steady speed on a straight and flat road and the distance/time measurement happens every second then the GPS speed is going to be pretty much the same as the actual speed in that second but a longer period of time to measure the distance travelled could make the GPS speed less accurate and doing 0-60 in 3 seconds will also mean that the indicated GPS speed is meaningless. As has also been said, if the road isn't straight the GPS only measures the distance/time between two points so assumes that you have gone between them in a straight line so this could underestimate your actual speed if there is a shrap bend involved.

Most speed cameras these days are laser and aren't averaging a speed measurement (well they are, but with thousands of measurements taken in 0.3 of a second and with the device rejecting readings that are out of a certain tolerance they can be regarded as instantaneous readings).

A speedomter is not allowed to under-read the speed but can over-read by up to 10%. There is a load of rubbish sometimes posted that says "If you were caught at 60 in a 50 your speedo must have been reading 66". That used to be the case but from my experience with cars from the last 20 years I think that speedomters over-read by a couple of mph and this is consitent throughout the speed range rather than being a perceontage above the actual speed as it may have been in the past.

grudas

1,224 posts

155 months

Wednesday
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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...

GPS receivers aren't bolted to buildings. As GPS stands for Gobal Positioning by Satellite, there's a clue in the name of where the signals come from. The receiver in the vehicle needs to receive signals from (I think) 3 satellites to get a "fix". The more satellites it can "see" the more accurate the indicated speed will be. 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. Of course, if you are doing a steady speed on a straight and flat road and the distance/time measurement happens every second then the GPS speed is going to be pretty much the same as the actual speed in that second but a longer period of time to measure the distance travelled could make the GPS speed less accurate and doing 0-60 in 3 seconds will also mean that the indicated GPS speed is meaningless. As has also been said, if the road isn't straight the GPS only measures the distance/time between two points so assumes that you have gone between them in a straight line so this could underestimate your actual speed if there is a shrap bend involved.

Most speed cameras these days are laser and aren't averaging a speed measurement (well they are, but with thousands of measurements taken in 0.3 of a second and with the device rejecting readings that are out of a certain tolerance they can be regarded as instantaneous readings).

A speedomter is not allowed to under-read the speed but can over-read by up to 10%. There is a load of rubbish sometimes posted that says "If you were caught at 60 in a 50 your speedo must have been reading 66". That used to be the case but from my experience with cars from the last 20 years I think that speedomters over-read by a couple of mph and this is consitent throughout the speed range rather than being a perceontage above the actual speed as it may have been in the past.
Spot on. Too many fairytales.

Super Sonic

2,612 posts

41 months

Wednesday
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douglasb said:
. As GPS stands for Gobal Positioning by Satellite, there's a clue in the name of where the signals come from.
Doesn't stand for that at all.
The 10%+2 is guideline only, if you get a ticket, saying "I was within the guideline" is no defence. If you constantly try to keep to eg. 78 in a 70, you need to concentrate on the speedo, or there's a good chance you could inadvertently go a tiny bit faster and get a ticket.

Super Sonic

2,612 posts

41 months

Wednesday
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TB404 said:
Hello people,
I want to confirm (or otherwise) my understanding of the subject.
the GPS speed shown in Waze is the actual speed that a speed camera would use to clock me?
is this correct? am I missing something?

Thank you,
No, the speed cameras don't rely on GPS, they use a laser. GPS can under read, as others have pointed out.

Edited by Super Sonic on Wednesday 22 March 06:57

Fore Left

1,204 posts

169 months

Wednesday
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douglasb said:
GPS receivers aren't bolted to buildings. As GPS stands for Gobal Positioning by Satellite, there's a clue in the name of where the signals come from. The receiver in the vehicle needs to receive signals from (I think) 3 satellites to get a "fix". The more satellites it can "see" the more accurate the indicated speed will be. 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. Of course, if you are doing a steady speed on a straight and flat road and the distance/time measurement happens every second then the GPS speed is going to be pretty much the same as the actual speed in that second but a longer period of time to measure the distance travelled could make the GPS speed less accurate and doing 0-60 in 3 seconds will also mean that the indicated GPS speed is meaningless. As has also been said, if the road isn't straight the GPS only measures the distance/time between two points so assumes that you have gone between them in a straight line so this could underestimate your actual speed if there is a shrap bend involved.
GPS stands for Global Positioning System.

Most phones update once per second. The distance travelled in that second is used to calculate speed. They do not wait for you to travel a measured distance or calculate over a longer time period. The error from going round a typical corner is insignificant. Driving round a track however you need a GPS receiver that updates at least 10 times a second.


OutInTheShed

4,338 posts

13 months

Wednesday
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douglasb said:
.....

GPS receivers aren't bolted to buildings.....
There are loads of GPS receivers bolted to buildings.
Often used for time references.

dontlookdown

1,287 posts

80 months

Wednesday
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douglasb said:
GPS receivers aren't bolted to buildings.
Surprisingly enough, there are plenty of GPS receivers attached to fixed structures like building these days. They are used in various automated building management and monitoring systems.

The posters point was that even GPS receivers that do not move can still record one or 2 mph - GPS speed measuring is not without error.

I usually split the difference between GPS and Speedo when driving.

PorkInsider

5,577 posts

128 months

Wednesday
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Super Sonic said:
The 10%+2 is guideline only, if you get a ticket, saying "I was within the guideline" is no defence.
No one in England has ever got a NIP for less than 10%+2 though, have they?

E-bmw

7,835 posts

139 months

Wednesday
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TB404 said:
I notice that on several different cars Waze reports about 10% lower GPS speed than the car's speedometer, I understand that's intentional. is this correct?
Incorrect WAZE will report up to 10% HIGH over the speedo reading.

TB404 said:
the GPS speed shown in Waze is the actual speed that a speed camera would use to clock me?
Incorrect, cameras don't use GPS they are calibrated using a calibrated car speedo or similar, so can also have apparent errors.


TB404 said:
On top of that we have the 10%+1 rule for speeding, which means one can drive 23mph in a 20mph zone )for example) and you won't get a ticket until you got to 24mph 'real speed' - i.e. GPS.
Also incorrect, the numbers mentioned are GUIDELINES, and while I am not suggesting you WILL get done for 31 in a 30, you CAN get done for it.

Edited by E-bmw on Wednesday 22 March 09:11

siremoon

65 posts

86 months

Wednesday
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[quote=douglasb
GPS receivers aren't bolted to buildings.

[/quote]

Wrong. GPS receivers can be used to provide an accurate time reference. My former employer had many equipment sites with GPS receivers fitted to provide such a reference.

vroomtshh

9 posts

67 months

Wednesday
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PorkInsider said:
No one in England has ever got a NIP for less than 10%+2 though, have they?
Never mind, internet says it didn;t happen

Edited by vroomtshh on Thursday 23 March 14:38

IJWS15

1,359 posts

72 months

Wednesday
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The key fact is that devices using GPS do not measure instantaneous speed, they measure average speed and the time interval over which that average is measured will depend on the device. GPS works on an accurate time signal to give a location, your device calculates the average speed between locations.

Other than average speed cameras enforcement devices use instantaneous measurement so your GPS showing 34mph doesn't mean you were not doing 35mph when they pinged you.

Your speedo will read high simply because, by law, it cannot read low so the manufacturers set them to read a little high (i.e. manufacturing tolerance on a speedo won't be +/- 1mph, it will be -3 to -1mph - both a range of 2mph).

I once saw a speed reading of over 700 mph (and it was rising by over 50 mph per second) on a TomTom, in a Honda Accord on the A38 near Burton-on-Trent so GPS devices can be wrong.

Craig_suke

28 posts

53 months

Wednesday
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vroomtshh said:
I got done for 64 in a 60 coming across the A66. Took the speed awareness course rather than points
Any proof of that one? 🥱