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anjomi
Occasional Visitor
Joined: Dec 07, 2007
Posts: 14
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 Posted: Fri Jan 18, 2008 3:27 am Post subject: |
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I'm sure the difference between Speedo and GPS is regional.
When I drive my car, or hire a car in the UK then the speedo is about 3-6mph lower than the GPS at 70mph. When I hire cars in the US, the Speedo and GPS are always nearly the same (within 1mph).
I saw someone quote in this thread that UK speedos are calibrated 5% + 2mph lower than true speed. It would appear that US speedos aren't.
I've haven't taken my TomTom into Europe or Asia yet. Has anybody got any info on these regions? |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Fri Jan 18, 2008 11:48 am Post subject: |
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| jspencer wrote: | | Skippy wrote: | | Therefore, if you reduce the diameter of the wheel (ie, by deflating a pneumatic tyre), then the circumference will reduce. |
That statement is true, but only if the diameter is reduced all the way around. If you have a flat the diameter has only been reduced in the vertical axis. The question you have not answered is whether the diameter increases in the horizontal axis to compensate for the distortion in the vertical axis. If it does, then your statement is meaningless. |
To make it clearer, replace "reduce the diameter of the wheel" with "reduce the rolling radius of the wheel", where the rolling radius is measured at the point where the tyre contacts the road.
I think the analogy of a wheel made out of sponge is the best one....
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Gone fishing! |
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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: Fri Jan 18, 2008 12:01 pm Post subject: |
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I was ALMOST at the stage of making one and videoing it!
Wife caught me with bath sponge in one hand and scissors in the other.... |
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Oldboy
Pocket GPS Moderator
Joined: Dec 08, 2004
Posts: 10641
Location: Suffolk, UK
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| Posted: Fri Jan 18, 2008 12:02 pm Post subject: |
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| Andy_P2002 wrote: | | Wife caught me with bath sponge in one hand and scissors in the other.... |
Who was worried the most .... you for getting caught, or the wife because she didn't know what you were up to? 
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Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1120 |
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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: Fri Jan 18, 2008 12:06 pm Post subject: |
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She's used to me and my little "projects" by now!  |
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Antoeknee
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Joined: Jul 29, 2005
Posts: 267
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| Posted: Fri Jan 18, 2008 1:04 pm Post subject: |
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M8TJT I don't think Mike was saying tyre pressure was a factor in the speed displayed by your TT. He was just saying that the technology exists for the TT to be able to dislapy other information such as tyre pressure.
Your absolutely correct that tyre pressure etc will not be an issue the TT will give truish indication of your speed. Fairly obvious really as rolling radius will vary from car to car depending on wheel and trye size. |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Fri Jan 18, 2008 1:40 pm Post subject: |
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| Andy_P2002 wrote: | I was ALMOST at the stage of making one and videoing it!
Wife caught me with bath sponge in one hand and scissors in the other.... |
ROFLMAO
    
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Gone fishing! |
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Robin2
Lifetime Member
Joined: Nov 24, 2003
Posts: 1441
Location: Swansea
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| Posted: Fri Jan 18, 2008 2:21 pm Post subject: |
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| anjomi wrote: | I'm sure the difference between Speedo and GPS is regional.
When I drive my car, or hire a car in the UK then the speedo is about 3-6mph lower than the GPS at 70mph. When I hire cars in the US, the Speedo and GPS are always nearly the same (within 1mph).
I saw someone quote in this thread that UK speedos are calibrated 5% + 2mph lower than true speed. It would appear that US speedos aren't.
I've haven't taken my TomTom into Europe or Asia yet. Has anybody got any info on these regions? |
The European construction and use regulations specify that speedos on cars for the European market must be between 0 and 10% of the true speed, not what the other poster quoted. Presumably it is different in the US
Incidentally, transmission damage in 4WD vehicles with uneven tread depths on either side is a real issue. It's not my field of expertise, but I guess it might be something to do with the heat generated in the type of limited slip differential which is commonly used.
Ooooh I do love this thread whenever it comes up (which seems to be quite often!!!)
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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M8TJT
The Other Tired Old Man
Joined: Apr 04, 2006
Posts: 10118
Location: Bexhill, South Sussex, UK
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| Posted: Fri Jan 18, 2008 7:45 pm Post subject: |
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| Antoeknee wrote: | | M8TJT I don't think Mike was saying tyre pressure was a factor in the speed displayed by your TT. He was just saying that the technology exists for the TT to be able to dislapy other information such as tyre pressure. |
Yep, Thanks for that. The was more implied than explicit
It also appears that Robin2's speedo reads somewhere between zero and seven MPH when he is doing an indicated 70 on his GPS. I always said, 'Trust the GPS' 
Oh, and by the way 'Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.' See http://www.gpsinformation.net/main/gpsspeed.htm
Can your wheel/gearbox driven, mechanically displayed (mostly) beat that then????? You may note that this statement says horizontal speed, so hills make no difference. But tyre pressure does to your speedo. |
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LesP2008
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Joined: Apr 05, 2006
Posts: 524
Location: Herts
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| Posted: Fri Jan 18, 2008 8:06 pm Post subject: |
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Correct me if i am mistaken but i always thought that UK speedos have to be calibrated above the true speed.
A police friend tells me that it is actually illegal to have one calibrated any differently.
Like, how would they know, they tell you your speed was 50mph in a 30mph, etc.
Come on Darren, you must have some input here. |
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Robin2
Lifetime Member
Joined: Nov 24, 2003
Posts: 1441
Location: Swansea
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| Posted: Fri Jan 18, 2008 9:38 pm Post subject: |
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| GO-GO-910 wrote: | Correct me if i am mistaken but i always thought that UK speedos have to be calibrated above the true speed.
A police friend tells me that it is actually illegal to have one calibrated any differently. |
That's exactly what I said. If you are travelling at a true 70mph, your speedo must show between 70 and 77. If it reads 69, it's illegal
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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LesP2008
Lifetime Member
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Joined: Apr 05, 2006
Posts: 524
Location: Herts
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| Posted: Fri Jan 18, 2008 9:47 pm Post subject: |
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| Sorry Robin, i must have overlooked that. No offence. |
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tgold
Regular Visitor
Joined: Nov 07, 2004
Posts: 98
Location: UK or USA from time to time.
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| Posted: Sat Jan 19, 2008 1:13 am Post subject: |
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| Robin2 wrote: | | The European construction and use regulations specify that speedos on cars for the European market must be between 0 and 10% of the true speed, not what the other poster quoted. |
Community Directive 97/39/EC, incorporated into UK C&U regulations by SI, requires that speedometers read no less than the true speed and no more than 110% of true speed + 4km/h.
Tony |
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anjomi
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Joined: Dec 07, 2007
Posts: 14
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| Posted: Sat Jan 19, 2008 4:03 pm Post subject: |
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I guess an important question to ask is what are the speed cameras callibrated to? I presume it must be the true speed (as given by the GPS).
I've heard, that they give you a 5% margin, so in a 70mph zone, does this mean they won't trigger until around 74mph true speed? If this is the case, then by my speedo I could go through them at nearly 80mph and not get caught.
Of course its not worth 3 points to test this theory... |
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Antoeknee
Lifetime Member
Joined: Jul 29, 2005
Posts: 267
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| Posted: Sat Jan 19, 2008 6:35 pm Post subject: |
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| anjomi wrote: | I guess an important question to ask is what are the speed cameras callibrated to? I presume it must be the true speed (as given by the GPS).
I've heard, that they give you a 5% margin, so in a 70mph zone, does this mean they won't trigger until around 74mph true speed? If this is the case, then by my speedo I could go through them at nearly 80mph and not get caught.
Of course its not worth 3 points to test this theory... |
Might be true.
I regularly drive A1 and drive past speed cameras in the 50 zones with 52 shown on TT and 56-57 shown on speedo.
Done this for couple of years and not had any nasty mail. |
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