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Accuracy of GPS Speed indicators
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MrRee4U
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Joined: Jun 11, 2005
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Location: Walsall - England

PostPosted: Wed Jun 15, 2005 9:15 pm    Post subject: Accuracy of GPS Speed indicators Reply with quote

I have just purchased the Medion MD95900 unit, maps by navman.

What I wondered could anyone / would anyone care to comment on the likely accuracy of the speed indicator when in use locked onto 6 - 9 sats.

What are anyones thoughts, good bad, accurate +/- 10% or what

Cheers

A Question
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Oldboy
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PostPosted: Wed Jun 15, 2005 9:36 pm    Post subject: Reply with quote

I've seen a figure of 0.3% quoted somewhere.
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Pocketgps
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PostPosted: Thu Jun 16, 2005 2:35 am    Post subject: Reply with quote

You need a level and stright road and a constant speed to get the best accuracy.
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StevenGourlay
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PostPosted: Mon Jun 27, 2005 8:53 am    Post subject: Reply with quote

they are accurate you will see that they give you a lower speed than what your card indicates but this is due to cars set normally 5mph greater.
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hertz
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PostPosted: Wed Jun 29, 2005 11:31 pm    Post subject: Reply with quote

I have discovered that they vary with different cars. My Peugeot 306's speedo is spot on, but as Steven Gourlay says, many cars speedos read up to 5mph faster than actual speed. My Freelander (previous one and present one) both indicate that the speedo is 5mph faster than my TomTom shows, and a Nissan Micra I once borrowed was also 5mph ambitious. At least, it reduces the chance of getting your photo taken!
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Skippy
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PostPosted: Wed Jun 29, 2005 11:51 pm    Post subject: Re: Accuracy of GPS Speed indicators Reply with quote

MrRee4U wrote:
What I wondered could anyone / would anyone care to comment on the likely accuracy of the speed indicator when in use locked onto 6 - 9 sats.


Lots of people have asked this one. The GPS manufacturers generally quote 0.1 MPH accuracy on flat ground, at constant speed and direction for several seconds and with a good sat lock.

Yes, most car speedos will read up to 10% high (you wouldn't want it to read too low, would you?)
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mburton2
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PostPosted: Thu Jul 14, 2005 7:39 am    Post subject: Reply with quote

Mine seems to vary....

When I was using my old Navman/Smart ST, it was reading APPROX 10mph lower than the speedo at an indicated 75mph

My current Globalsat BT338/TT5 is much closer to the speedo on the (same) car.......about 5mph less than the Navman woudl read.

The Legal requirement for a (OEM) cars speed accuracy is:
+10%+4kph

ie its POSSIBLE to indicate 114kph, but only be doing 100kph
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gtvv6
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PostPosted: Fri Jul 15, 2005 11:08 pm    Post subject: Reply with quote

Thing is, car speedo accuracy may not be uniform accross the scale, my Alfa shows +4mph at 70, but at 30 it's spot on, so don't assume you can drive at 35mph speedo reading safely in a 30 zone.

I borrowed my son's Garmin Vista a while ago when I went away for a weekend, he has since accused me of doing some obscene speed as shown in the trip computer (which I forgot to clear), he isn't buying my story about poor GPS accuracy! He takes my lectures on safe driving with a huge pinch of salt now Embarassed
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mburton2
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PostPosted: Sat Jul 16, 2005 10:12 am    Post subject: Reply with quote

Yip you are right, the accuracy CAN vary depending on you actual speed, what I quoted was the MAX in-accuracy allowed.
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Donald
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Location: Yeovil, Somerset

PostPosted: Fri Jul 22, 2005 2:05 pm    Post subject: Re: Accuracy of GPS Speed indicators Reply with quote

MrRee4U wrote:
I have just purchased the Medion MD95900 unit, maps by navman.

What I wondered could anyone / would anyone care to comment on the likely accuracy of the speed indicator when in use locked onto 6 - 9 sats.

What are anyones thoughts, good bad, accurate +/- 10% or what

Cheers

A Question


I notice no one queries the velocity figure of the appropriate receiver. I haven't checked, but is this part of the equation?

I have noticed that in my car, a Ford Focus Ghia, the speedo lags the GPS indication by about 5mph. Sad In a friend's Merc, the Speedo tracks the GPS spot on up to 80mph (No idea of above that 'coz the driver didn't 8O ...)

The other thought is that the caesium clocks on the satellites are incredibly accurate and logic tells me that if that is the source of the speed calculations, then the GPS speed indication will also be very accurate. :D

That said, if a car with a blue lamp on top or if TT indicates a SafetyCam, I use may car speedo as the definitive. I would hate to have to argue “M’lord, … My TomTom…… GPS…. US military… Caesium Clock….. Guilty but insane!!”
Crying or Very sad
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Skippy
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PostPosted: Fri Jul 22, 2005 9:43 pm    Post subject: Reply with quote

gtvv6 wrote:
some obscene speed as shown in the trip computer


Police Naughty. Laughing

My GPS was showing a max speed of 602 MPH on it's trip computer for a while. No, I wasn't driving - I was a passenger... In a Boeing 747-400. Wink
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GaryC
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PostPosted: Fri Aug 19, 2005 9:17 pm    Post subject: Reply with quote

With my speedo reading 30 the SatNav reads consistently 27mph and at 70 straight and level only 64mph by GPS
If GPS is as accurate as said and given I solely bought the SatNav to keep me on the straight and narrow as far as speed and speed cameras are concerned, it came as quite a surprise to find according to GPS it's unlikely I have exceed a speed limit by more than the 'allowed' 10% at the most...I better find another use for it or what a waste of money Crying or Very sad
The one thing that worries me is if I am travelling in a convoy though a 30 limit, my speedo is reading 30. Question is then is GPS so accurate especially in built up area, or is everyones speedo set accurately 10% fast!?
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Oldboy
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PostPosted: Fri Aug 19, 2005 10:43 pm    Post subject: Reply with quote

GaryC wrote:
it came as quite a surprise to find according to GPS it's unlikely I have exceed a speed limit by more than the 'allowed' 10% at the most

With the data you quoted, and the GPS being more accurate than the speedo reading, if you have been driving 70 on your speedo, how could you ever have been breaking the 70 speed limit?
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Pocketgps
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PostPosted: Sat Aug 20, 2005 12:52 am    Post subject: Reply with quote

GaryC wrote:
The one thing that worries me is if I am travelling in a convoy though a 30 limit, my speedo is reading 30. Question is then is GPS so accurate especially in built up area, or is everyones speedo set accurately 10% fast!?


The UK requires a speedo to be accurate or 'fast' -0%, +10% at 30 mph.
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GaryC
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PostPosted: Sat Aug 20, 2005 1:57 am    Post subject: Reply with quote

Oldboy wrote:
GaryC wrote:
it came as quite a surprise to find according to GPS it's unlikely I have exceed a speed limit by more than the 'allowed' 10% at the most

With the data you quoted, and the GPS being more accurate than the speedo reading, if you have been driving 70 on your speedo, how could you ever have been breaking the 70 speed limit?


It was rather tongue in cheek but that's exactly my point given it would seem speedo's in general read 10% plus and you don't get nicked I understand until 10% over then in a 30 limit speedo readings are 36+ and 70 near 85 seems more reasonable.
Don't get me wrong I am not being sanctimonious far from it, it's just trying to understand where I stand and how to get on with my journey quickly and safely without falling foul of the money making machines and most important keep my eyes on the road...not on the speedo.
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