Thread: How accurate are GPS speedos

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.

Originally Posted by Crackerjack

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.

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.

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.

Originally Posted by 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.

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

Originally Posted by V8Ian

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.

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

Originally Posted by p38arover

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.