Thread: Car Speedo Vs GPS

I use the GPS in the truck and 4WD rather then the speedo and have checked them against radar and the GPS is spot on.

Originally Posted by DJ's Timber

I use the GPS in the truck and 4WD rather then the speedo and have checked them against radar and the GPS is spot on.

Originally Posted by DJ's Timber

I use the GPS in the truck and 4WD rather then the speedo and have checked them against radar and the GPS is spot on.

I have checked mine against a radar too - not that I meant to, it just worked out that way

I have a Scangauge fitted to the van and it shows the same speed as the GPS but the speedo always reads 5 -6 kms over..
Never tested against radar

In my experience, car speedometers always read 5-10 per cent high, stock standard, of course.

I've checked that against several GPS' over time, and always found it to be the case.

You can further check it by timing yourself though those specially marked out sections 5 km on some highways (here in Qld, at least), but you'll find they will agree with your GPS rather than the speedometer.

Ian

Car manufacturers are concerned that variability in speedometers could result in a legal situation whereby a car owner would blame a speeding fine on them so they deliberately make their speedos read between 5 and 10% less than the real deal.

BTW I have checked my GPS out at a range of speeds over 500 km/hr. Yes they work in a passenger jet - you know the flight data screen, height above sea level, outside temp etc, well the GPS read exactly the same as what was on that screen - of course it would because that is also what these planes use. I often fly with one of my GPSs as I like to know what is on the ground we are passing over when I look out the window.

I always thought that the manufacturers had our welfare at heart Bob, in deliberately keeping us under the speed limit.

Silly me! Your explanation is far more logical.

Hum not that I have a GPS yet or even used the mobile phone one. BUT if police radar has been proved to be inaccurate due to such as solar spikes, wind, rain, the two blokes siting in the vehicle and moving about, rate of data feed then what makes you think/believe GPS from a satellite is any better

Point in my corner is such as GPS coordinates given from ground to US Pilots and error in air to ground missiles using all the same technology.

Originally Posted by wheelinround

Hum not that I have a GPS yet or even used the mobile phone one. BUT if police radar has been proved to be inaccurate due to such as solar spikes, wind, rain, the two blokes siting in the vehicle and moving about, rate of data feed then what makes you think/believe GPS from a satellite is any better

Point in my corner is such as GPS coordinates given from ground to US Pilots and error in air to ground missiles using all the same technology.

The absolute positional error (ie to target a specific location on the earth) of consumer level GPS is at the metre+ level and deliberately kept so by the US military, but the relative positional error is usually at the cm level. Hence a car travelling at 60 km/hr (16.7 m/s) over one second from a notional 0 +/- 10 cm to 16700 +/- 10cm has a "change in position uncertainty" of 20 parts in 16700 0r 2 parts per 1670 or 0.1%. The faster you go the lower the uncertainty.

Even at slow (walking pace) speeds ordinary GPS is highly accurate.

However they are not perfect. I have seen them spike around by +/- 10% or fail completely when navigating around narrow streets lined with tall buildings and they do not of course work in tunnels - but on ordinary city/suburban streets and highways and country roads they are accurate to at least 1 km/hr.

Back in 01-02 when I worked for the WA Camera section I actually looked in to it.
At the time the ADR's stated that a car speedo was accurate if reading + or - 10% - that meant that when your speedo read 100kph you could be doing anywhere between 90 & 110 yet your speedo was considered accurate according to the ADR's.
At the time WA speed camera's were set at 8kph over the speed limit, IE 108 in a 100 zone, so in effect the police were demanding that you monitor your speed to a higher degree of accuracy than your speedo would allow according to the ADR's.

I think it's time to revisit the whole speeding issue, I've seen both sides of it - from working for the WA police and from being a habitual speeder since I got my licence which will be 26 years ago come May.
The fact is that it is too easy to lose your licence, in Queensland, getting booked 4 times for 13k over is all it takes - and in todays cars 13k over is nothing, my car is only a 6 cylinder falcon and that does 140-150 without trying.
I think we need to look at a sliding scale, doing 15 over on a long straight stretch of the Bruce highway (say Anzac Ave up to the end of the dual carriage way) is no big deal and perfectly safe. Doing 15 over past a school or in a suburban 60 is bloody stupid and highly dangerous.
Both the same offence with the same penalty yet one is a lot more dangerous.
Something to think about.
Paul

Originally Posted by smidsy

Back in 01-02 when I worked for the WA Camera section I actually looked in to it.
At the time the ADR's stated that a car speedo was accurate if reading + or - 10% - that meant that when your speedo read 100kph you could be doing anywhere between 90 & 110 yet your speedo was considered accurate according to the ADR's.
At the time WA speed camera's were set at 8kph over the speed limit, IE 108 in a 100 zone, so in effect the police were demanding that you monitor your speed to a higher degree of accuracy than your speedo would allow according to the ADR's.

I don't know when this kicked in but now the rules are that cars off a showroom floor cannot read over the limit so most manufacturers set the speedo ~5% below the nominal value.
If you choose to modify the vehicle that results in higher readings that is of course not the manufacturers fault.
The last 4 cars I have owned have been 05, 08, 09, and 12 models and were all between 8 and 3 % under the limit.

RE: Sliding scale based on a judgement call on location, Hummmm . . . what about weather conditions, traffic density, vehicle condition and driver experience, I reckon all these are just as relevant. My concern is it could be open to all sorts of interpretation mainly from the copper that is booking you, maybe he doesn't like the cut of your job or something.i

With regard to the original question, I believe that it would depend on your particular "GPS" and its intended use.

I have a popular brand 2nd gen navigator that I use occassionally to locate addresses that I have not been to previously. I use it occasionally because it is a POS. Approaching an intersection where I have to turn, I can be at the point where I have to turn and it will tell me to turn in 50metres. I can drive along a freeway with a 100kph speed limit and a 50KPH feeder/distributor road 20m to the side and it will beep speed limit alarms because it thinks I am on the feeder/distributor. As I approach an intersect and slow to stop, it tells me I am doing 10KPH or more at the point when I stop, i.e there is appreciable lag in the hardware/software.

Navigators tend to locate you a couple of times a second, and derive a speed for you by dividing the distance travelled between samples by the time between samples, with an appropriate multiplier to convert to KPH or MPH as user selected. This seems to work while travelling in a straight line on a level surface. Climb or decend a slope, go around a curve etc, and the actual distance travelled and instantaneous speed are different from the point to point straight line calculated by the unit. The difference is not a lot, but can be enough to get you pinged if you rely absolutely on the navigator. I do use the GPS to check the accuracy of the speedo in the car, but as an occasional check on a long straight level stretch of road without much traffic, so that I have the time to drive, check speedo, check navigator etc repeatedly over at least a km. Using this method, I believe my speedo is accurate to within my ability to interpret a wide needle on a medium sized scale. The car is 18 years old and fitted with the same brand/style/size tyres that it left the factory with, and a mechanical speedo.

There are of course a variety of "GPS" systems ranging from bushwalking/cycling navigators, through the automotive navigtators, to highly accurate static 3D units for land surveying and dynamic 3D units for aircraft navigation, which provide altitude, location, and speed information for navigation. Depending on intended use and product price, performance as a speed indicator would be quite variable accross the the whole "GPS" range.

My GPS is a mid range model Garmin - bought because I was (still am) new to Brisbane and don't know my way round.
The biggest annoyance I found with it is that it would send me in to a tunnel and then refuse to work due to the lack of signal in the tunnel.
At this stage I've actually stopped using it unless I'm in a rush, I find that using a paper road map and having to think instead of follow instructions is beneficial in terms of learning the roads.

What about older cars that are booked for doing over the limit?

Remember when Steve Bracks decided that the level of tolerance was
to be 1kmph? Twit!!!

This whole issue will remain a thorn in the side of motorists while ever
there is money to be had for the pickings by moronic politicians.

I am always annoyed by the slogan that "Every K over is a killer."
What arrant nonsense!!! At that rate we should all be dead several times over!!!

Rant off!