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teamgers Occasional Visitor
Joined: Dec 20, 2007 Posts: 10
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Posted: Sat Dec 29, 2007 4:08 pm Post subject: Speed accuracy |
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Hi, can anyone tell me which is the more accurate, the speed indicated on your sat nav? or, the speed indicated on your speedo in the car?
Many thanks |
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PaulB2005 Pocket GPS Moderator
Joined: Jan 04, 2006 Posts: 9323 Location: Durham, UK
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Posted: Sat Dec 29, 2007 4:12 pm Post subject: |
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Moved to more appropriate forum
The Sat Nav will be more accurate as speedos tend to under read the speed. |
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Reap Occasional Visitor
Joined: Dec 15, 2007 Posts: 9
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Posted: Sun Dec 30, 2007 10:51 am Post subject: |
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Quote: | The Sat Nav will be more accurate as speedos tend to under read the speed. |
Speedos certainly under read, but surely GPS suffers from lag and going up or down hill will make it read slower than it is. Also an area of bad satellite reception may effect it? |
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mikealder Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Sun Dec 30, 2007 11:06 am Post subject: |
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The GPS speed is only updated once per second so there will always be a slight lag, as for going up and down hills degrading the accuracy yes it will do, but try working out by how much and you will find it is a very small degradation, the GPS is always more accurate than the crude device the vehicle manufacturer supplies (exept in tunnels 0 - Mike |
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Anita Pocket GPS Moderator
Joined: Mar 15, 2006 Posts: 3219 Location: Windlesham, Surrey
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Posted: Sun Dec 30, 2007 12:02 pm Post subject: |
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PaulB2005 wrote: | The Sat Nav will be more accurate as speedos tend to under read the speed. |
I think you'll find speedos tend to over read, i.e. the speed shown is faster than the actual speed, in my Ka by about 10%. _________________ Anita
TomTom VIA 135 - App 12.075
UK map 1130.12368
Samsung Galaxy S21 |
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PaulB2005 Pocket GPS Moderator
Joined: Jan 04, 2006 Posts: 9323 Location: Durham, UK
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Posted: Sun Dec 30, 2007 1:42 pm Post subject: |
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Quote: | I think you'll find speedos tend to over read |
Hmm... Too much xmas cheer i fear.... Of course you are right. |
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Robin2 Lifetime Member
Joined: Nov 24, 2003 Posts: 1441 Location: Swansea
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Posted: Sun Dec 30, 2007 8:25 pm Post subject: |
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Satnav speed is very accurate if, and only if, you are travelling at a steady speed, on the level and in a straight line. Under those circumstances you can rely upon it to much better than 1 mph to calibrate your speedo. Cars sold in Europe must have speedos that are zero to 10% fast. To allow for changes due to tyre wear, under inflation etc most manufacturers aim for a few percent fast, to ensure that if using the speedo to keep to the limit there is no possibility of unknowingly exceeding it. My Ford shows 72 mph at a satnav speed of 70, my wife's Citroen shows 73. My previous car, a VW, was 74! I use the satnav to set up my cruise control to a GPS speed of 70 on the motorway.
The Construction and Use regulations don't cover speeds as low as 20 mph, so car speedos may be much less accurate at these speeds. With the increasing number of 20 mph zones a GPS check may be even more important to avoid accidentally exceeding the limit.
Robin _________________ TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used |
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if1977 Lifetime Member
Joined: Nov 30, 2007 Posts: 649 Location: Doncaster
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Posted: Sun Dec 30, 2007 11:29 pm Post subject: |
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I would also like to say that if you change your wheel size i.e. fitting alloys that are bigger or smaller will also give you different readings on your speedo as the rotation of the speedo pick-up sensor in the gearbox changes it's revolutions due to the different sized wheels . I know that the speedo heads are not accurate anyway in most cars but if you buy a car that is fitted with non-manufactured alloys this will make it worse. So if you buy the car like this don't assume that it is all down to the speedo. _________________ NEW Tomtom GO 500 (hope it is good)
iphone (running all day, doesn't like it)
XL LIVE IQ Routes (Got stolen and miss it)
GO 510 (in drawer now, time for upgrade) |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Mon Dec 31, 2007 12:31 am Post subject: |
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[quote="Reap"] Quote: | going up or down hill will make it read slower than it is. |
Quick bit of Pythagoras...
On a 1 in 10 hill, for a horizontal travel of 100yds that the GPS registers, you actually travel 100.04998750624609648232582877001 yds - meaning an error of -0.05% on the speed indicated. Hardly significant! |
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xda Lifetime Member
Joined: Mar 11, 2004 Posts: 1199 Location: Park Gate
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Posted: Mon Dec 31, 2007 5:37 am Post subject: |
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if1977 wrote: | I would also like to say that if you change your wheel size i.e. fitting alloys that are bigger or smaller will also give you different readings on your speedo as the rotation of the speedo pick-up sensor in the gearbox changes it's revolutions due to the different sized wheels. |
Most cars these days have electronic or digital speedometers. So changing to Alloys etc. just needs the local dealer to electronically re-calibrate it.
In my owners manual it clearly states that if having the optional alloys fitted then the speedometer must be re-calibrated by your Ford Dealer. _________________ Graham.
TT Go720, App:9.510(1234792.1) OS:842337
GPS: V1.20, Boot: 5.5279, Home: V2.9.5.3093
Map: Europe V910.4892
Map: Europe_Truck V870.3421, Kingston 8GB SD
Nokia 925 Windows 8 |
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philpugh Lifetime Member
Joined: Dec 28, 2005 Posts: 2003 Location: Antrobus, Cheshire
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Posted: Mon Dec 31, 2007 6:21 pm Post subject: |
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[quote="Andy_P2002"] Reap wrote: | Quote: | going up or down hill will make it read slower than it is. |
Quick bit of Pythagoras...
On a 1 in 10 hill, for a horizontal travel of 100yds that the GPS registers, you actually travel 100.04998750624609648232582877001 yds - meaning an error of -0.05% on the speed indicated. Hardly significant! |
Whilst agreeing it is insignificant ... and being very pedantic
The 'base' of your triangle needs to be curved to match the spheroid of the datum (eg WGS84). So the amount of under-reading will depend upon if you are going N-S or E-W (and directions in between) and where on the surface of the earth you are travelling. _________________ Phil |
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robertn Frequent Visitor
Joined: Feb 06, 2005 Posts: 564
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Posted: Mon Dec 31, 2007 9:45 pm Post subject: |
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One point to consider is the algorythms used by the GPS manufactures. The GPS receiever reports a speed to the Sat Nav, which is probably pretty accrutate, BUT will depend on the mode of the device - usually Automotive, which means it is averaged over the last 3 or 5 seconds. Do this test - driving along at a constant speed, stop suddenly (When it's safe)
As soon as ou are stopped, watch the speed on the GPS..... I bet you are still doing 30mph.
Then the Sat Nav software converts it to Miles/Hour, and either rounds of truncates it. Heres the thing - we don't know which, so 60 could be between 60 and 61, or 59.5 and 60.5.
Slightly less pedantic than the previous posts , but at least it does not change with tyre wear. |
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Snudge Lifetime Member
Joined: Aug 22, 2007 Posts: 211 Location: Peterborough
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Posted: Tue Jan 01, 2008 2:47 am Post subject: |
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Andy_P2002 wrote: |
Quick bit of Pythagoras...
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I had a new slide-rule for Xmas and I can't get the figure that Andy does? |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 01, 2008 1:00 pm Post subject: |
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philpugh wrote: |
The 'base' of your triangle needs to be curved to match the spheroid of the datum (eg WGS84). |
You're right of course!
I remember at school a long time ago, having to work out the curvature required on a snooker table to stop all the balls running into the middle!
snudge wrote: | I had a new slide-rule for Xmas and I can't get the figure that Andy does? |
Also very possible!
I think I added a nought that I shouldn't have...
I did it for a 10x1 triangle which gives 10.04987562112089027021926491276 and scaled up (badly!)
How about 100.4987562112089027021926491276? |
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mikealder Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Tue Jan 01, 2008 1:05 pm Post subject: |
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And I thought you had the correct answer the first time due to the curved base of the triangle used - Mike |
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