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PHILIP2908
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Joined: Apr 03, 2005
Posts: 17
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 Posted: Wed Sep 24, 2008 2:54 am Post subject: Speed on Tom Tom - accutate ?? |
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I've had a Tom tom ONE for a couple of years now and have always noticed the the speed shown is about 5mph less than the speedo on my Passat.
Having recently driven 2000mls in the USA in a Dodge Charger - (truly awful car), but this had an integrated GPS system that provided internet radio on the move (fabulous) and I think must have driven the speedo, as for the first time the speed shown there, matched that on my Tom Tom which I took with me.
The question is: does anyone trust the speed shown on their SatNav when driving past speed cameras, SPECS etc rather than the dashboard instrument and not got a ticket??
I'm not brave enough yet - unless you tell me otherwise !!
Philip 2908 |
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biggles150
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Joined: Jan 09, 2006
Posts: 101
Location: Eltham, London
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| Posted: Wed Sep 24, 2008 3:24 am Post subject: |
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Hi PHILIP2908,
I'm going to start off with a bit of a moan. you joined this forum in 2005, and I can't believe you haven't come across one of the dozens of topics on this subject, moan over 
Suffice it to say that the GPS speed indication is more accurate than your speedo, which is designed to read approximately 10% over your actual speed.
I hope this is not going to be the start of an already well over discussed topic.
USE THE SEARCH FUNCTION.
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Calomax
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Joined: Sep 30, 2005
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Location: St Martin's, Guernsey
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| Posted: Wed Sep 24, 2008 8:37 am Post subject: |
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And if you find the other threads you will find that there is no corresponding law in the US that speedos have to overread so their's are usually accurate as you found on your trip.
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Darren
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Joined: 11/07/2002 14:36:40
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Location: Hampshire, UK
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| Posted: Wed Sep 24, 2008 8:44 am Post subject: |
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| Calomax wrote: | | And if you find the other threads you will find that there is no corresponding law in the US that speedos have to overread so their's are usually accurate as you found on your trip. |
This is incorrect. US Federal standards also allow a maximum speedometer error of 5%.
Whilst you can find some cars anywhere that will read accurate, most won't as manufacturers inevitable err on the side of caution.
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Darren Griffin |
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Darren
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Location: Hampshire, UK
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| Posted: Wed Sep 24, 2008 8:46 am Post subject: Re: Speed on Tom Tom - accutate ?? |
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| PHILIP2908 wrote: | | Having recently driven 2000mls in the USA in a Dodge Charger - (truly awful car), but this had an integrated GPS system that provided internet radio on the move (fabulous) and I think must have driven the speedo, as for the first time the speed shown there, matched that on my Tom Tom which I took with me. |
In the US they have two satellite radio services, Sirius and XM. They have been very successful in the US because it is much easier to provide radio via satellite than maintain a network of terrestrial broadcast antennas. They share nothing with GPS except that they use satellite, different satellites, different orbits etc.
The speedo in the Dodge was just unusually inaccurate.
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Calomax
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Joined: Sep 30, 2005
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Location: St Martin's, Guernsey
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| Posted: Wed Sep 24, 2008 8:47 am Post subject: |
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Sorry Darren, I meant they don't have to overread as the do have to in Europe
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Darren
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Location: Hampshire, UK
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| Posted: Wed Sep 24, 2008 8:51 am Post subject: |
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| Calomax wrote: | | Sorry Darren, I meant they don't have to overread as the do have to in Europe |
Ah ok but in Europe they don't have to either. Legislation states they must be in the range 0% to +10%.
As they must not read under, manufacturers err on the side of caution most often and so configure them to over-read. They don't have to, it's just easier for them to do so.
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peterc10
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Joined: Aug 21, 2005
Posts: 1761
Location: Kent, England
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| Posted: Wed Sep 24, 2008 11:34 am Post subject: |
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| Darren wrote: | | As they must not read under, manufacturers err on the side of caution most often and so configure them to over-read. They don't have to, it's just easier for them to do so. |
Not a case of ease but necessity.
Speedos work by counting revolutions of the drive shaft and converting that to distance. But distance is a function of outer diameter of the wheel on the road and that can change a lot by factors such as make of tyre, tread depth and tyre pressure (not to mention things like using different size wheels and tyres on same model range but using standard speedo!)
So the range is needed to cope with all this and still make sure it does not under-read.
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henryg
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Joined: Jul 05, 2005
Posts: 78
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| Posted: Wed Sep 24, 2008 11:43 am Post subject: |
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Personal experience with my current and previous cars, seems to suggest that both speedos overestimated the speed by 3-4mph rather than a speed dependent percentage.
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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: Thu Sep 25, 2008 4:19 pm Post subject: |
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| biggles150 wrote: |
I hope this is not going to be the start of an already well over discussed topic.
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Please?
It's all been said so many times already, as Biggles says try the search function. |
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daimlerv8
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Joined: Sep 24, 2008
Posts: 14
Location: LEEDS
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| Posted: Sat Sep 27, 2008 7:18 am Post subject: |
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On a friends calibrated speedo........it read 4MPH faster than the Tom Tom
both the go 500 and the go 910 for an exact speed reading i trust the Tom Tom v my speedo. |
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