How accurate are GPS speedos

[Jeff]

My GPS has a speedo, but I have no way of telling how accurate it is. In theory they should be accurate, bit I'm not so sure.

Jeff

[Crackerjack]

I treat my Tom Tom speed as being accurate, the way the satellite system works, the speed has to be accurate as it is constantly working out where it is and giving a distance between 2 points, therefore giving an accurate speed between those two points, all the speedos of the cars I have had it in are inaccurate by up to 4 mph.

[p38arover]

all the speedos of the cars I have had it in are inaccurate by up to 4 mph.

Both our Subaru Foresters (2005 and 2009) read 118km/h at 110 km/h (by the GPS).

My P38A is spot on at 110km/h.

[Blknight.aus]

its accurate to the radius of the Dilution of position circle/sec


IF your GPS has that feature enabled it will give you a circular error of position number express that per second and thats how far it can be out.

since mine manages to get that down to under 15 cms on a good day I figure its pretty accurate.

[rmp]

GPS receivers are very accurate speedometers, provided that:

1. You have a 3D fix (see the receiver's documentation)
2. The vehicle is travelling at a constant speed on the flat

there is no need to measure the time from point to point. That will be less accurate than simply reading from the screen, because there will be a position error on both start and end points, and a small timing error.

The GPSR works out its speed by Doppler shift relative to the satellites, not by working out a position, noting a time, then doing the same again.

Most vehicles' speedos are set by the manufacturer to over-read by 2-4%, eg you may be doing 110 but it would read 113.

[V8Ian]

GPS receivers are very accurate speedometers, provided that:

1. You have a 3D fix (see the receiver's documentation)
2. The vehicle is travelling at a constant speed on the flat

there is no need to measure the time from point to point. That will be less accurate than simply reading from the screen, because there will be a position error on both start and end points, and a small timing error.

The GPSR works out its speed by Doppler shift relative to the satellites, not by working out a position, noting a time, then doing the same again.

Most vehicles' speedos are set by the manufacturer to over-read by 2-4%, eg you may be doing 110 but it would read 113.
Note 3d is a quadulation, not a triangulation, meaning it is comparing information from 4 sattelites.

[rmp]

Note 3d is a quadulation, not a triangulation, meaning it is comparing information from 4 sattelites.

Correct. The 3"d" is dimensions, not satellites. However, I don't say to use 4 because that would require an explanation of the different stages of satellite lock acquisition, and merely seeing 4 birds may not mean a 3D lock is acquired. If the 3D fix is shown then the speedo should be all good.

[TwoUp]

If your GPS can lock in to Land Base Stations and provides real time (kinematic) you will be OK, otherwise the GPS does not know how much in error it is, even if it displays an error reading.

Regards,
PeterW

[subasurf]

Both our Subaru Foresters (2005 and 2009) read 118km/h at 110 km/h (by the GPS).


My 2001 Forester is absolutely spot on with my TomTom. :D

[austastar]

Hi,
I read some where that it is trilateration (http://www.google.com.au/#hl=en&source=hp&q=define+trilateration&meta=&aq=0&oq=define%3Atrilaterat&fp=2c80c4fbb94c2fe1) that gps works on.
cheers

[Lotz-A-Landies]

My RRc has a increasing accuracy as the speed increases. At 60KPH by the speedo the GPS (NavMan and Garmin) read 65KPH, at 80KPH by the speedo it's 83KPH by the GPS an all the readings correspond at 110KPH.

When you go through the radar speed checks on Victorian main roads, my NavMan is either spot on or +/- 1KPH which will be difference in averaging up or down of the 2 devices and the time delay between reading the sign and the GPS.

The result is that I trust my GPS' but the Law in NSW doesn't recognise them as accurate.

Diana

P.S. I find the NavMan very innaccurate in the M5-East tunnel! :D :D

[e3j]

Just an aside,

I always thought it was illegal - at least in Australia - for a speedo to read low. Eg read 100k when you are really doing 105k. But my Disco2 reads 110k when my GPS says 113k. And no it's not the GPS as I recently upgraded it and both the old and new read this.

No problem if you are aware of it, but just wondered if it's common in LR's?

[V8Ian]

Just an aside,

I always thought it was illegal - at least in Australia - for a speedo to read low. Eg read 100k when you are really doing 105k. But my Disco2 reads 110k when my GPS says 113k. And no it's not the GPS as I recently upgraded it and both the old and new read this.

No problem if you are aware of it, but just wondered if it's common in LR's?
Putting larger tyres on will do that.

[isuzurover]

No problem if you are aware of it, but just wondered if it's common in LR's?

I have found the opposite. On both my IIA and 110, going from (factory) 32" diameter tyres to 33" tyres has made the speedo almost 100% accurate.

On a trip across the Nullarbor in the 110 we checked it against a GPS, and over 100 gps km my ODO showed 105.

While we are on this topic, how accurate are GPS units at measuring altitude these days??? (assuming clear sky above you)

[austastar]

Easily seen by putting the gps down on the beach.
Not very! Ten meters would be average on mine.
Needs birds overhead and on the horison to get a good calculation.
cheers

[richard4u2]

i have the mio 220 when it had the 2007 map on it , it was spot on with the speedo but now it has the 2008 map on it , it is 3 ks out

[Grockle]

Nearly all car speedo's read 10% fast on std tyres