Accuracy of satnav speed reading, compared to speedometer

[NLB]

My cheapo TomTom satnav gives a speed reading, as do most I believe. Does anyone know how accurate this is? I am inclined to think that, unless there is a deliberate “falsification” algorithm in the system, for satnav based speed measurement to work at all, it has to be fairly accurate. It is, of course, amazing that it can be made to work at all, given that it is derived from a signal from a satellite around 12,500 miles out, picked up by an aerial that can fit in a box the size of a packet of fags... but we have all become blasé about this stuff.

The other issue which follows is, I always knew speedometers over-read, but it seem to be a bit more than I had always allowed for. The one on the BMW reads about 7% over, as far as I can tell – the satnav says 65, the speedo says 70 (the speedo on the Alpine either reads or not, as it feels like...). Therefore, the normal cruising speed of (harrumph, mumble) mph, is actually only a little bit over the posted limit... So, going by the satnav reading, I am tending to drive just that bit faster..... is this wholly irrational?

[Ben Hughes]

My Focus seems to be about 5-7mph out between 50mph and 85mph. In those useless 50 mph average speed checks on motorways, I can drive along with the speedo showing 55 and the TomTom bang on 50.

[NLB]

Ah - exactly the same here.... and, apparently, everyone's speedo is over-reading by about the same amount, because they are all doing 46mph....

[davido140]

indeed, I've been trusting tom tom over my cars speedo for several years now.

Not been caught out by cameras or the BiB yet... (probably tempting fate now)

[Ash 996 GT2]

Satnav wins everytime.

It has to be accurate to be able to calculate arrival times for a start.

It also has to be accurate because it needs to know where you are and when to tell you to turn.

I go by my Satnav speed, and yes, my speedo is out by about the same as yours.