Accurate speed reading

Mickser_52

Can anyone tell me which is more accurate, the speed shown on my car speedometer or the speed shown on my gps screen? Travelling on the motorway at 120 km/h (on speedometer) the gps was showing 115 km/h. When I (accidently!;)) travelled at 125 km/h the gps was showing 120 km/h?

trad

Speedometers are programmed to read fast so I would suspect the gps to be more accurate. You could always ask your local friendly traffic cop to give you a reading.

Absurdum

gps

Gumbo

i would also say GPS is more accurate.

Bards

has to be GPS, how else does it know where you are, when to turn, time to detination etc etc.

corktina

GPS

i reently bought a indic8tor unit to use as a speedo in one of my classics and ALL my cars read over by 5 to 10% Thats 6 cars....

the only time there is a lag with the GPS is when accelerating or decelerating and its fractional

101sean

All car speedos over read by 5% or more. If you ever go to the UK with loads of speed cameras in road works especially the average speed ones traffic tends tends to go at 3-5mph under the limit because of this. Same with drivers here who slam on the anchors as soon as they see a 50kph sign :rolleyes:.

GPS is accurate at steady speed (more than 5secs in a straight line) as are strangely the speedos in both my Land Rovers

Capt'n Midnight

As above , GPS usually have a readout once per second , there may be a lag too while it figures out what the satellites are saying, and if coverage is interrupted it may give inaccurate results till it locks on. But all that is worst case and on average it will be much better.


Car speedos also have to cope with tire size, if you change your tires / rims it can affect the speedo .


How accurate are those speed signs at the side of the road ?

And why don't they put them beside speed cameras ?
You wouldn't have to spend ages worrying about whether you were just under the limit or not

Mickser_52

Thanks everyone for confirming what I already suspected. I have also found similar info from other websites. If I knew a friendly traffic cop I could ask him but not sure if they exist:). Could I argue this case if I was summonsed for speeding? Not that it is likely to happen .

Dum_Dum

GPS is less accurate because it cannot measure any distance travelled perpendicular to the earth's surface; i.e. going up and down hills.

Consider climbing steps; GPS will measure your lateral displacement only, which is always less than the actual distance - the hypotenuse. Therefore GPS reading < actual speed with the difference proportional to the degree of inclination.

L1011

trad wrote: »

Speedometers are programmed to read fast so I would suspect the gps to be more accurate. You could always ask your local friendly traffic cop to give you a reading.

They're not specifically "programmed to read fast", they just legally aren't allowed under-read.

The speedo in my Citroen C4 is accurate to within 1km/h of the GPS up to 140km/h or so. The speedo in my Panda 100 is woefully wrong above 100km/h, will be displaying 180+ by the time you get to 160km/h.

Alun

Dum_Dum wrote: »

GPS is less accurate because it cannot measure any distance travelled perpendicular to the earth's surface; i.e. going up and down hills.

Consider climbing steps; GPS will measure your lateral displacement only, which is always less than the actual distance - the hypotenuse. Therefore GPS reading < actual speed with the difference proportional to the degree of inclination.

You'd have to be going up one serious hill to make very much difference. Take a 1 in 10 incline ... sqrt(10*10 + 1*1) = 10.05, or 0.5% more distance travelled.

And that's assuming it actually calculates speed in that way. It may well do, but in addition the GPS receiver modules themselves, as opposed to the part (i.e. computer) that does all the display and further processing, are capable of determining velocity by using doppler techniques. This is a) true velocity, not speed measured along the horizontal, and b) has very low requirements as far as satellite geometry goes, less than you'd normally expect for a good 3-D positional fix.

Also this 1 second lag thing that everyone mentions is purely down to the rate at which the GPS module sends it's data to the processor that does all the display and further processing. Depending on the capabilities of the module in question it could easily send that data more often, or indeed less often.

Alun

MYOB wrote: »

They're not specifically "programmed to read fast", they just legally aren't allowed under-read.

Some manufacturers clearly are choosing to deliberately add in a bias in order to make absolutely certain they aren't under reading though.

My Focus's OBC has an undocumented mode where I can display all kinds of stuff including speed. That consistently displays to within 1 km/h or so of the GPS whereas the analogue display consistently shows a higher reading by about 5%. And before anybody says anything about 'needles' being inaccurate, modern speedos are driven by stepper motors controlled by the dashboard computers, not the wobbly induction motor setups of old, so are just as accurate as digital speedos.

dymonaz

Dum_Dum wrote: »

Therefore GPS reading < actual speed with the difference proportional to the degree of inclination.

This is correct, even if, as Alun says, it's just 0.5% of a difference.

However, if GPS is less then actual, than the speedo, as evidenced, is even further less than actual.

The 1s lag shouldn't make any difference to the actual speed reading, especially if you're travelling on a motorway at a constant pace.

Bruthal

Alun wrote: »

Some manufacturers clearly are choosing to deliberately add in a bias in order to make absolutely certain they aren't under reading though.

My Focus's OBC has an undocumented mode where I can display all kinds of stuff including speed. That consistently displays to within 1 km/h or so of the GPS whereas the analogue display consistently shows a higher reading by about 5%. And before anybody says anything about 'needles' being inaccurate, modern speedos are driven by stepper motors controlled by the dashboard computers, not the wobbly induction motor setups of old, so are just as accurate as digital speedos.

This is the true reason, cars are intentionally reading on the fast side so you cant be going faster than it says, so your clock has to show you going a fair bit over the limit before you actually are, as for gps reading more inaccurate as you drive on an incline, they measure speed in 3 dimensional space so i would of thought even if your going straight up it would be accurate,
A sat nav needs at least 4 satellite fixes to give a location, and 4 satellites will give velocity no matter what direction it is, a minimum of 4 satellites is needed to give a fix in 3 dimensional space, if only 3 are connected to the gps system then there are 2 possible locations on the earths surface that would be possible from the distance to any 3 satellites, the 4th gives the last distance to show what actual point your at, so i would of thought travel in any direction would of been measure accurately. Interesting point though, but what reference would tell the satellites the sat nav is on an incline,,,, as in if someone is driving north in dublin on a horizontal road, what about someone in africa travelling in the same direction, they would be moving uphill in reference to the dublin driver even if on a horizontal road in their location, the fact is the speed is measured by changing distance to at minimum of 4 satellites and so the speed horizontal or vertical will be measured accurately.

Bruthal

Mickser_52 wrote: »

Can anyone tell me which is more accurate, the speed shown on my car speedometer or the speed shown on my gps screen? Travelling on the motorway at 120 km/h (on speedometer) the gps was showing 115 km/h. When I (accidently!;)) travelled at 125 km/h the gps was showing 120 km/h?

the gps is the accurate reading. Cars are biased to show faster than your actually going.

Bruthal

Mickser_52 wrote: »

Thanks everyone for confirming what I already suspected. I have also found similar info from other websites. If I knew a friendly traffic cop I could ask him but not sure if they exist:). Could I argue this case if I was summonsed for speeding? Not that it is likely to happen .

You cant argue the case if summonsed for speeding, thats the whole point as to why cars read slightly above your actual speed, it gives you no excuse if done for speeding. If limit is 100, and your done for going 110, well your speedo would of said at least 115, so it eliminates the excuse of only being barely over the limit.

Mickser_52

The point I was trying to make in relation to arguing a speeding charge was, could I argue in court that the satnav was indicating a speed of 120 km/h while the speedomoter was at 125 km/h or so. In the unlikely event that I showed up as exceeding the speed limit.

Bruthal

Mickser_52 wrote: »

The point I was trying to make in relation to arguing a speeding charge was, could I argue in court that the satnav was indicating a speed of 120 km/h while the speedomoter was at 125 km/h or so. In the unlikely event that I showed up as exceeding the speed limit.

We could probably argue anything in court, they would just say the speedometer which is part of the car is what we should observe, you would only have a case if could prove the speedometer read 110 but your going 120, if the limit is 120 as on a motorway and you were doing 130 and charged, your speedo would of said 135 or so, and satnav would say 130, so what could you argue, as the speedo over estimates speed slightly, which should help keep drivers below limit,

Maybe you meant if your sat nav showed 120 but they claimed your going 130? id say then you could argue about the sat nav indication, but you would have to show their speed trap was in error, interesting one alright

corktina

Mickser_52 wrote: »

The point I was trying to make in relation to arguing a speeding charge was, could I argue in court that the satnav was indicating a speed of 120 km/h while the speedomoter was at 125 km/h or so. In the unlikely event that I showed up as exceeding the speed limit.

they'd use that as evidence that you were knowingly speeding Id say

Bruthal

corktina wrote: »

they'd use that as evidence that you were knowingly speeding Id say

Yes thats the whole reason the speedo on cars is set to read slightly above actual speed, so you cant claim you were only barely over the limit,
The car can not be travelling above the speedo reading by law i think.

KevR

Capt'n Midnight wrote: »

How accurate are those speed signs at the side of the road ?

From my experience these are not very accurate. Very hit and miss also.

unkel

MYOB wrote: »

They're not specifically "programmed to read fast", they just legally aren't allowed under-read.

Yup that's it - it's a European directive.

MYOB wrote: »

The speedo in my Panda 100 is woefully wrong above 100km/h, will be displaying 180+ by the time you get to 160km/h.

Once search is back up, you can search motors for previous threads about this. Until then, you'll have to take my word for it that the speedo is allowed to show any speed between the actual speed and the actual speed + 10% + 4km/h

This means your Panda displaying 180km/h when the actual speed is 160km/h is fully compliant. Just.

(160 + 10%*160 + 4 = 180km/h)