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Stormin
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Joined: Dec 31, 2004
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 Posted: Wed Jan 05, 2005 5:33 pm Post subject: Speed Indicator |
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| How true is the speed reading on the tomtom go, as my speedo on the car would appear to be 4mph out |
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buckman
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| Posted: Wed Jan 05, 2005 6:50 pm Post subject: |
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| Mine too! |
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tomtombmth
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| Posted: Wed Jan 05, 2005 7:32 pm Post subject: |
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alot of people say the gps speed is more accurate than your speedometer. How true this is I dont know.
I have had quite a few different Navigation systems and there always have been 2-4mph difference. |
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rkm_hm
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| Posted: Wed Jan 05, 2005 8:32 pm Post subject: |
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Car exaggerometers often give a high reading - both to enhance the notion of 'performance' and also to make it less likely that you will get done for speeding by accident.
Under steady speed conditions, I would believe the GPS! |
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MikeB
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Location: Essex, UK
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| Posted: Wed Jan 05, 2005 9:20 pm Post subject: |
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| tomtombmth wrote: | alot of people say the gps speed is more accurate than your speedometer. How true this is I dont know.
I have had quite a few different Navigation systems and there always have been 2-4mph difference. |
If you ever go abroad to Europe or the USA you wiil find that the speed on the Speedo exactly matches that of the GPS when you are travelling at a steady pace. It is the UK specific cars that give false (over) readings.
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Darren
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| Posted: Wed Jan 05, 2005 10:29 pm Post subject: |
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UK (and possibly EU) legislation requires that speedometers are either 100% accurate or read up to 5% over the true speed. Most manufacturers err on the side of caution and set them to read high.
Given a level and straight stretch of road then a GPS Speed calculation is more accurate comeing close to true speed, climbing or descending, travelling around curves etc will reduce the accuracy.
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rkm_hm
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Location: Warwick, UK
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| Posted: Wed Jan 05, 2005 11:04 pm Post subject: |
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| Darren wrote: | | climbing or descending . . . will reduce the accuracy. |
That's presumably because the GPS will measure the horizontal component of your motion - whereas you're actually travelling along the hypoteneuse of a right-angled triangle (in side elevation)?
If so, on a 1in 6 hill (approx 10 degrees) you'll actually be going about 1.5% faster than the GPS says [cos(10degrees)=0.985 ish]. That's the steepest incline on which you're likely to be going at a speed worth worrying about.
On a 1 in 12, the error is only about 0.4% |
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Old_Phil
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| Posted: Wed Jan 05, 2005 11:41 pm Post subject: |
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The statement that GPS systems are more accurate than your car speedometer is almost always true - in the UK the law states that speedo readings must not exceed the true speed, so the manufacturerers deliberately set our speedos to read high, typically around 5% at 70 mph.
Think about the technology used in your car. It's based on counting the number of revolutions of your wheels, which will have a varying circumference as the tyre wears over time. Hardly surprising it's difficult to establish a reliable and accurate true speed indication. It's amazing that they are only 4mph out at 70!
So, if your speedo reads 70 mph you are almost certainly actually travelling at somewhere between 65-68 mph. You will absolutely not be travelling at 70 mph.
Your GO is much more accurate than your speedo - a bonus.
BTW - it's not a good idea to adjust your speed to take account of this knowledge, if it's new. Continue driving at a steady 30, 40, 50, 60 or 70mph as indicated by your speedo safe in the knowledge that you will not actually be exceeding those speeds. |
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DavidW
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Joined: 17/05/2003 02:26:21
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Location: Bedfordshire, UK
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| Posted: Wed Jan 05, 2005 11:44 pm Post subject: |
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UK and, I believe, EU regulations on speedometers is that they must not show a speed slower than your true speed, but can show a speed of up to 10% faster than your true speed. That means that most speedometers tend to be rather 'fast', though not necessarily in a linear way (the amount that your speedo is 'fast' may depend on the speed).
Tachographs, where they have to be fitted by law, have to hold to a much tighter specification - but I don't know, off the top of my head, what that specification is.
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Blue_Bear
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Location: Warrington, Cheshire
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| Posted: Thu Jan 06, 2005 12:02 am Post subject: |
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I have a TTG & Road Angel II and they both display the exact same spped, where as my Speedo is about by various percentages depending upon my speed. Aledgedly at 135 in a Honda Accord it reads 121 on the GPS Systems.
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rkm_hm
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Location: Warwick, UK
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| Posted: Thu Jan 06, 2005 12:09 am Post subject: |
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| Old_Phil wrote: |
Think about the technology used in your car. It's based on counting the number of revolutions of your wheels, which will have a varying circumference as the tyre wears over time. Hardly surprising it's difficult to establish a reliable and accurate true speed indication. It's amazing that they are only 4mph out at 70! |
I don't think that's necessarily true - because a tyre is not a rigid body, but distorts as each section of tread comes into contact with the road.
I'm sure I've heard it claimed by tyre manufacturers that there is very little change in revs/mile between a new and worn tyre because what matters - in radials at any rate - is the circumference of the belt, which doesn't change. So the 4mph error at 70mph is constant, and deliberate!
Roger |
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DocL
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Joined: Nov 15, 2004
Posts: 81
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| Posted: Thu Jan 06, 2005 12:13 am Post subject: |
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| DavidW wrote: | | UK and, I believe, EU regulations on speedometers is that they must not show a speed slower than your true speed, but can show a speed of up to 10% faster than your true speed. That means that most speedometers tend to be rather 'fast', though not necessarily in a linear way (the amount that your speedo is 'fast' may depend on the speed). |
I agree with David here, and as an additional piece of information here is a posting I put in the 'other' forum, on the same subject;
"Right folks, I generally agree with g0sbv, but I thought the 'over-speed' error was limited to +10%; there is 0% under-read allowance for obvious reasons. All normal car speedos etc. should comply with these accuracy limits.
However, Police patrol cars have regularly calibrated speedos, and a friendly 'Blackrat' (not ex-!) placed his Tom Tom in his patrol car and when he compared the Tom Tom speed reading with the car's speedo reading (recording them both on the in-built video set-up) he found that Tom Tom was pretty well identical to the calibrated speedo. While the car's speedo gives an instantaneous reading, Tom Tom does not; merely a refreshed reading every so often. I'm not sure what this refresh rate is, but appears to be ~10secs or so.
The speedo in my Octavia seems to over-read by ~7% throughout the speed range."
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rkm_hm
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Joined: Nov 28, 2004
Posts: 532
Location: Warwick, UK
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| Posted: Thu Jan 06, 2005 12:17 am Post subject: |
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| MikeB wrote: | | If you ever go abroad to Europe or the USA you will find that the speed on the Speedo exactly matches that of the GPS when you are travelling at a steady pace. |
I was trying to work out how the speedo calibration of my car would mysteriously change when I took it abroad!
Then I realised that you were probably referring to hiring a 'local' car, rather than taking your own UK-spec car with you! |
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