To those of you with GPS speedos

benjus

This article on GPS speed measurement is quite interesting: http://www.aprs.net/vm/gps_cs.htm

It was written while Selective Availability (SA) was still in use, which made the signal only accurate to around 100m for civilian use. It suggests that without SA, the reported speed is normally accurate to within 0.07mph (and accurate to within 1mph with SA).

marlot

Strider590 wrote: »

Speedo dial = 78(ish)
OBDII reading directly from the cars ECU = 71
GPS = 70

Conclusive proof that the dial has been calibrated to show a higher than actual reading, whilst the car's ECU knows it's exact speed to be much lower.

I found that on my Mondeo - the car knew the real speed, but the speedo was deliberately reporting a little faster.

On my current landcruiser, speedo says 60, real speed is 56.

pinkteapot

2010 Mondeo:

Speedo 70mph = sat nav 67mph

I tend to drive to the sat nav reading TBH, especially in motorway roadwork 50 limits. Where I get to 50mph on the sat nav then hit cruise control.

pinkteapot

marlot wrote: »

I found that on my Mondeo - the car knew the real speed, but the speedo was deliberately reporting a little faster.

Because it's illegal for a speedo to under-read, so they err on the side of caution and make them over-read...

Lum

2010 Ford Mondeo 2.0 TDCi

Speedo 75 - GPS 70.

GPS device is an Origin B2 with the external antenna stuck in the back window.

pinkteapot

Lum wrote: »

2010 Ford Mondeo 2.0 TDCi

Speedo 75 - GPS 70.

Our 2010 Mondeo 2.0 TDCi is the same (probably not surprising really).

Gloomendoom

pinkteapot wrote: »

Because it's illegal for a speedo to under-read, so they err on the side of caution and make them over-read...

IIRC the over read has to be within 10%

adouglasmhor

vax2002 wrote: »

It measures flat distance, unfortunately roads have hills and dips, so sat navs are often a bit low.

Does it really? I thought it measures 3d vector speed by taking bearings from 3 or more satellites simultaneously, also some fitted ones use dead reckoning from engine pulses or other fitted input from the vehicle, but I am willing to be proved wrong do you have any up to date links etc.?
http://www.satnavforensics.com/how-sat-nav-works.php

blacksta

My sat is about 2 to 3 miles behind the Speedo

Ford ka

TrickyWicky

The_ICT_Engineer wrote: »

Actual I own Dutton Commander, Which I can just drive into the thames.

Apart from showing off, what useful information have you contributed so far to the ops question?

None.