Accuracy of GPS Speed indicators

astrocompass · Posted: Wed Apr 05, 2006 9:48 pm Post subject:

An apology to Skippy - you're absolutely right. Did a bit of digging in the last few days (sorry to resuscitate an old thread, but it might help us all) and a couple of things emerged.

1. Wet Film (Gatso, Truvelo) Cameras WILL flash even if there's no film, or if the film has ended. However, some will only flash once rather than twice, so that might be a bit of a giveaway some of the time.
2. Cameras are often replaced with "dummies" so that, to the untutored eye and for one who has time to look, it seems as if they may be live. Part of the psyops to keep us all on our toes, and to keep those submissions coming in.....

Regret having to draw out this thread, but felt that Skippy was due an apology Embarassed

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ScottMoffatt · Occasional Visitor Joined: Oct 02, 2005 Posts: 10

I transfer my Medion PNA250 between various vehicles and find that the speedo accuracy varies widely.
Most are over-reading in the region of +8-10%, but the worst is my current works vehicle - an Astra 1.7CDTI Van - which over-reads by about 12%.

I also have a kit car - and as part of the SVA test the speedo calibration is checked. The limits are as mentioned previously in this thread.
I had to adjust the calibration at the test centre - and after 3 attempts achieved spot-on accuracy at 30, 40, 50, 60 and 70mph. I watched the tests being carried out.
I could have gone for a "safe option" over-reading, but it is a good thing to have a measuring device that is accurate!

However I have found that (seemingly dependent on routing calculations / POI warnings), the displayed speed can take a few seconds to update.
I'd agree with a previous post that the displayed GPS speed is only valid under level road steady-state conditions.

Scott.

mrgrumpy23 · Occasional Visitor Joined: Mar 26, 2006 Posts: 18

Durotriges · Posted: Wed Feb 28, 2007 4:12 am Post subject:

I've often wondered about the accuracy of the GPS device speed readings. If they are accurate to within a small percantage of the vehicle's true speed, how might this be explained:

At an indicated 70mph in my car, my friend's TomTom Go300 says I'm doing 65mph and my girlfriend's Road Angel Navigator 6000 says I'm doing 68mph.

I'm wondering which one is telling the truth!

Mike

:D

Skippy · Posted: Wed Feb 28, 2007 2:43 pm Post subject:

Durotriges · Posted: Thu Mar 01, 2007 2:57 pm Post subject:

Hello Skippy and thanks for the welcome :D!

Hahaha, I had to laugh about your last comment! So you must know her then! Laughing

The readings I got were at different times, but taken over a long journey each time - and were of a stable speed and direction (once up the M5 and once down the A30 to Plymouth). For each journey the discrepancy was constant, so I don't think it was a signal strength issue.

I've just got myself an iPaq h2210 running TomTom Navigator, so I'll see what that has to say for itself.

Mike

philpugh · Posted: Sat Mar 03, 2007 2:53 am Post subject:

Durotriges · Posted: Mon Mar 12, 2007 12:25 am Post subject:

Hello Phil.

Thanks for the information. I've since tried my TomTom running on my iPaq with my girfriend's Road Angel in the same car and the readings were the same for the speed.

I'll have to check my TomTom on iPaq against my friend's Go300 to see if there is a difference there.

Mike

robertn · Frequent Visitor Joined: Feb 06, 2005 Posts: 564

On your PDA I would log the raw GPS and see what the GPS is reporting in it's NMEA output, or use something like winfast navigator. The problem is that ROad Angel, TT, Navman etc can "play" with the numbers before they display it to you.

The most likely scenario as I see it is:
The car speedo is always optimistic.
Road Angel would tend to be a bit conservitive - speed it their busniess and they do not want to be blamed for a ticket when they said you were not speeding
TomTom - the speed display is an ancilary function to the core of providing Navigation - the probably take the raw GPS and just display it. I am cautious with my Navmans as they possible just truncate the speed - so 70.9 MPH is displayed as 70. Even if the round, 70.4 will display as 70 and still (technically) get you pinged. So I always drive at 69 (officer :D )

Durotriges · Posted: Mon Mar 12, 2007 9:16 pm Post subject:

Thanks for your reply :D. Logging raw GPS and NMEA things are beyond me at the moment, but that's something to learn about next!

I like your theory regarding the discrepancies. It would be interesting to find out what is actually happening.

Mike

gumbossos · Occasional Visitor Joined: Jun 21, 2007 Posts: 1

wrt car speedos, all cars have to comply to European law which states:
The speed indicated must never be less than the true speed....and
there shall be the following relationship between the speed indicated on the dial of the speedometer (V1) and the true speed (V2):

0 is less than or equal to V1-V2 which is also less than or equal to V2/10 + 4km/h

at all speeds above 40km/h

this is almost always checked with a GPS unit. it has to have an accuracy better than +/-1.0%

if that helps at all....

Durotriges · Posted: Thu Jun 21, 2007 4:20 pm Post subject:

That's interesting. Sometimes you get in a car and it feels as if you are going faster for the indicated speed. When this happens it's usually when the speedometer is accurate!

The last time this happened was when my friend bought an old Audi Coupe (1986). We checked it at various speeds against the mile markers that the police use on our local dual carriageway and it turned out to be reading accurately. Having used my TomTom GPS in the car it confirmed that the speedo reads accurately. Another really accurate speedo I found was on a Rover 827.

The worst I have come across are in Vauxhall Cavaliers; it was really noticeable when driving one, having been used to a different car. One that we measured against the mile markers was reading fast by 10% at an indicated 30mph, increasing to 15% at an indicated 80mph.

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