General Speedo accuracy

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General Speedo accuracy

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Jun 1, 2011
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Started using my TomTom today and noticed that the speedo in the 500 is way out. 56mph on the speedo was consistently showing as 50mph on the satnav.
I always thought that when I was in ECO it felt like I was the slowest vehicle on the road. Now I know it's true!

I've never seen this variation on any of the cars I've had over the past 10 years. Has anyone else noticed this?
 
Every car I've ever had has been out. GPS is more accurate as it actually tracks your movement across the earth, as opposed to reading the speed from the wheels. It's also why people use GPS for top speed runs as the speedo isn't accurate enough.

Speedometers in this country are also meant to be set up to show a speed that's higher than the actual speed of the car. This is because of various laws relating to being caught by speed cameras and so on. Your car will always over-read, and more so if the wheels are changed for a different size.

Here's what a quick search throws up:

http://forums.whirlpool.net.au/archive/881459
http://www.tomtomforums.com/tomtom-one-start-ease-xl-xxl/22053-accuracy-vehicle-speed.html
 
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Yes, but just remember that GPS tracks you along the earth and the earth isn't flat.....

Neither is the orbit of the GPS satellites. They are more accurate than a speedometer and always will be, especially in the current world of 'it's not my fault, i'll sue you'!
 
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I've used mine in work van, my car, wife's car, and a hire van and all were out by nearly bang on 5mph (speedo's read lower)

Manufactures safety margin? :shrug:
 
Neither is the orbit of the GPS satellites. They are more accurate than a speedometer and always will be, especially in the current world of 'it's not my fault, i'll sue you'!
I agree that they're more accurate that the speedo, but they shouldn't be used as an absolute measure of speed.
 
Well, the Earth may be "curved" and satellite orbit may be comparable, but the more immediate issue is whether you are going up or down a hill at all as this affects the result noticeably. If read over time, GPS is very good for the domestic curious in my opinion.
 
Well, the Earth may be "curved" and satellite orbit may be comparable, but the more immediate issue is whether you are going up or down a hill at all as this affects the result noticeably. If read over time, GPS is very good for the domestic curious in my opinion.

This is a popular misconception, but given that the GPS signal is generated in medium earth orbit, just over 20,000 kms from the earth's surface, unless you are falling from the top of Everest the percentage difference is minimal.

Garmin quote speed accuracies of around 0.1 to 0.5% under standard conditions.

Recent GPS units use Wide Area Augmentation Systems (WAAS) or European Geostationary Navigation Overlay Service (EGNOS) to provide accuracies of 1 - 3 metres.

It is easy to be sceptical about the abilities of GPS but modern units provide a degree of accuracy undreamt of 20 years ago and in all cases far more accurate than any but the Police calibrated speedometers.
 
This is a popular misconception, but given that the GPS signal is generated in medium earth orbit, just over 20,000 kms from the earth's surface, unless you are falling from the top of Everest the percentage difference is minimal.

Garmin quote speed accuracies of around 0.1 to 0.5% under standard conditions.

Recent GPS units use Wide Area Augmentation Systems (WAAS) or European Geostationary Navigation Overlay Service (EGNOS) to provide accuracies of 1 - 3 metres.

It is easy to be sceptical about the abilities of GPS but modern units provide a degree of accuracy undreamt of 20 years ago and in all cases far more accurate than any but the Police calibrated speedometers.

Cheers for that. I was pretty sure I have noticed a difference going over gradients at motorway speeds, observed by comparing RPM with a constant as possible speed. Extreme placebo effect of thinking I know the science! Next time I go for a long drive I will observe again and see if the result changes!
 
"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or [ame]http://en.wikipedia.org/wiki/ECE_Regulation[/ame] 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed." Hansard
 
This is a popular misconception, but given that the GPS signal is generated in medium earth orbit, just over 20,000 kms from the earth's surface, unless you are falling from the top of Everest the percentage difference is minimal.

Garmin quote speed accuracies of around 0.1 to 0.5% under standard conditions.

Recent GPS units use Wide Area Augmentation Systems (WAAS) or European Geostationary Navigation Overlay Service (EGNOS) to provide accuracies of 1 - 3 metres.

It is easy to be sceptical about the abilities of GPS but modern units provide a degree of accuracy undreamt of 20 years ago and in all cases far more accurate than any but the Police calibrated speedometers.

I don't see how the height of the satellites orbit makes a difference? Unless your GPS unit knows the gradient then it will simply assume that you're travelling on flat ground and tell you that you're going slower than you will be going.
 
Your observation of speed variation with gradient could occur if you only had a limited fix on say 1 or 2 satellites, but in practice this should never be the case under normal conditions. One of the other factors is that GPS receivers only sample the data intermittently ie they have a sampling rate of 1 second or less to give an average speed so that the display is not constantly changing by fractions. More professional receivers used for vehicle data logging have sampling frequencies of typically 100Hz
 
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Your observation of speed variation with gradient could occur if you only had a limited fix on say 1 or 2 satellites, but in practice this should never be the case under normal conditions. One of the other factors is that GPS receivers only sample the data intermittently ie they have a sampling rate of 1 second or less to give an average speed so that the display is not constantly changing by fractions. More professional receivers used for vehicle data logging have sampling frequencies of typically 10kHz

I have to say I don't quite agree with that, but then I'm just a born sceptic. Sure, GPS is definitely more accurate on flat ground than a speedo, but I don't think it should be used as an absolutel 100% cast iron measure of speed.
 
It only offers 99.5% accuracy in commercially available vehicle systems - but thats close enough for practical purposes.
There are many technical papers to substantiate the claims - I have to say I am not familiar with the accuracy of the GPS systems built into many smartphones for those who use them for navigation, but I would imagine they are at least equivalent to those used by TomTom
 
I don't see how the height of the satellites orbit makes a difference? Unless your GPS unit knows the gradient then it will simply assume that you're travelling on flat ground and tell you that you're going slower than you will be going.

I think he was pointing out that because the satellites are so far away from the Earth's surface, a gradient in the road will make minimal difference to the reading. Certainly not enough to be noticeable on the average sat nav!
 
I think he was pointing out that because the satellites are so far away from the Earth's surface, a gradient in the road will make minimal difference to the reading. Certainly not enough to be noticeable on the average sat nav!

But I was under the impression that GPS was so accurate that it would.....
 
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