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hebron38
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 Posted: Mon Jan 08, 2007 8:28 pm Post subject: Actual Speed |
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| I notice that there is a fairly substantial difference on the vehicle speed recorded on my TomTom Go 910 compared to the speedo. reading on the car. The speedo. is showing a faster speed than the TomTom. Which is more likely to be accurate? |
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Border_Collie
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Joined: Feb 01, 2006
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Location: Rainham, Kent. England.
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| Posted: Mon Jan 08, 2007 8:35 pm Post subject: |
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On a straight level road the GPS will be more accurate.
Car speedo's can over read by as much as 10% but are not allowed to under read.
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hebron38
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| Posted: Tue Jan 09, 2007 12:06 am Post subject: Actual Speed |
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| Thanks, at 70 mph on the Motorway with the TomTom the speedo was registering 78 mph ! |
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Robin2
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Location: Swansea
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| Posted: Tue Jan 09, 2007 12:16 am Post subject: |
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If you are driving at a steady speed, in a straight line and on the level, the indicated GPS speed will be accurate to better than 1 mph. But if, for example, you are driving up a 1 in 10 hill, TomTom will underestimate your speed over the ground (which is what safety cameras measure!) by approx 10%.
As Lost_Property said, European Construction and Use Regulations allow a tolerance from 0 to 10% in car speedos, and as the tyres wear, the wheels rotate faster for a given speed, so the speedo can over-read even more. The thinking behind the regulation is that if you drive at the limit according to your speedo, you cannot possibly be exceeding it.
Robin
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zogman
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| Posted: Tue Jan 09, 2007 12:19 am Post subject: |
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robin ,doesn't the tomtom take the altitude into consideration when making its calculations..
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Robin2
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| Posted: Tue Jan 09, 2007 12:43 am Post subject: |
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I don't think so - it seems to track the speed in 2D as seen from overhead. I have tested this out by driving at a GPS speed of 30mph on a hill near where I live, and then at a GPS speed of 30mph on the level. The speedo indications differ by about 10%
Robin
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TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
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Plus a lot of other PDA GPS kit, seldom used |
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Andy_P
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Joined: Jun 04, 2005
Posts: 19991
Location: West and Southwest London
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| Posted: Tue Jan 09, 2007 4:08 am Post subject: |
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 "speedometer"
There's at least 8 previous dicussions on this exact subject. |
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Skippy
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Joined: 24/06/2003 00:22:12
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Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Tue Jan 09, 2007 9:42 pm Post subject: |
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| Robin2 wrote: | | if you are driving up a 1 in 10 hill, TomTom will underestimate your speed over the ground (which is what safety cameras measure!) by approx 10%. |
Pythagoras said "the square of the hypotenuse is equal to the sum of the squares of the other two sides of the triangle".
So the actual difference travelled on a 1 in 10 gradient is the square root of (1 * 1) + (10 * 10) which is 10.05
So the error is actually in the order of 0.5%
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Border_Collie
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Joined: Feb 01, 2006
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Location: Rainham, Kent. England.
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| Posted: Tue Jan 09, 2007 9:46 pm Post subject: |
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Nice one Skippy, so....
| Quote: | | There was an Indian chief, and he had three squaws that he kept in three separate teepees. When he would come home late from hunting, he would not know which teepee contained which squaw, as it was very dark. One day, he went hunting and killed a hippopotamus, a bear, and a buffalo. He put a hide from each animal into a different teepee so that when he came home late, he could feel inside the teepee and he would know which squaw was inside. After about a year, all three squaws had produced children. The squaw on the bear had a baby boy; the squaw on the buffalo hide had a baby girl. But the squaw on the hippopotamus had both a girl AND a boy. So what is the moral of the story? The squaw on the hippopotamus is equal to the sum of the squaws on the other two hides. |
:P
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trog100
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| Posted: Tue Jan 09, 2007 9:51 pm Post subject: |
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he he he
trog |
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trog100
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| Posted: Tue Jan 09, 2007 9:56 pm Post subject: |
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does anyone remember the mackison maxims..
the eskimo who lit a fire in is canoe to keep warm..
u cant have your kyak and heat it..
the rabbit that fell in love with a vole but it idnt last..
a vole and his bunny are soon parted..
he he
trog |
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Border_Collie
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| Posted: Tue Jan 09, 2007 10:03 pm Post subject: |
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Skippy. Read carefully what Robin2 said, I think he's on to something.
| Quote: | | it seems to track the speed in 2D as seen from overhead |
If you imagine a stretch of road which is 4x long and rises 3x, the third side would be 5x, as per 'Pie Fag'. But looking from above i.e. 2D, the distance covered would appear to be 4x but is actually 5x i.e 25% difference.
Maybe why that's why TT sometimes routes off at a junction then immediately back on.
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Border_Collie
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| Posted: Tue Jan 09, 2007 10:11 pm Post subject: |
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Your as bad as me trog100 :P
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Robin2
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| Posted: Wed Jan 10, 2007 1:23 am Post subject: |
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Skippy's maths are correct and I was wrong  But the strange thing is that on this hill near where I live the speedo indication is about 2 or 3 mph lower than at the same GPS speed on the level!!! 
Robin
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Skippy
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Joined: 24/06/2003 00:22:12
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Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Wed Jan 10, 2007 1:34 am Post subject: |
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| Lost_Property wrote: | Skippy. Read carefully what Robin2 said, I think he's on to something.
| Quote: | | it seems to track the speed in 2D as seen from overhead |
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Yes, I agree with that. To take it to an extreme, if you drop your GPS off a cliff it won't register any speed at all for the vertical drop (if it still works when it hits the bottom). 
| Lost_Property wrote: | If you imagine a stretch of road which is 4x long and rises 3x, the third side would be 5x, as per 'Pie Fag'. But looking from above i.e. 2D, the distance covered would appear to be 4x but is actually 5x i.e 25% difference.
Maybe why that's why TT sometimes routes off at a junction then immediately back on. |
Interesting theory, but the problem is that the 3 4 5 triangle dimensions you quote the vehicle would have to climb or descend a 1 in 1.3 or 75% gradient which is practically impossible for a road vehicle.
A typical gradient for a steep hill is 1 in 4 or 25% which would cause a 3% error in the reading. However, you probably wouldn't want to do more than 30 MPH down a slope like this for fear or losing control of your vehicle.
In New Zealand there is a street which is a 1 in 2.66 or 38% gradient which claims to be the world's steepest paved street. It's bloody hard work to walk up too. 
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