As topic really.

Ive noticed that using 3 different GPS units the speed reported is often a few mph above my car speedo, and the difference increases with speed. So say the speedo says 30mph, the gps would would be 27mph At 70mph on the speedo, its more like 63-64mph!

I remember hearing that car speedos can be 10% out or something?

It cant just be my car though as it seems everyone does around about the same speed limit past cameras etc, suggesting either GPS tends to report slower speeds continuously, or car speedos overestimate the speed to make driving safer?

I figured a gps unit would be accurate if driven at a uniform speed on a straight stretch of road for a long time though?

Just curious which you guys would tend to trust. keep meaning to wait for one of those "Watch your speed" things that tell you your speed when you go above the limit, and do 31 through it and see what my GPS and speedo say?

Yes but Im talking about is say you were on a 10 mile straight stretch of road. The speedo is constantly on the 70 mark. So, youre at a constant speed. The GPS should surely over 10 miles be able to calculate current speed based on distance between 2 points over a certain time, and if that speed is constant, lag wouldnt factor in?

And with GPS units its Satellite->Box...theyre passive arent they, they dont send any info out?

on a motorway, your GPS will prob be more accurate As the software measures point A to point B,etc as it updates & works out your speed by distance travelled, lag time wouldnt really affect this... and your talking a split second anyway.


Cars are usually tuned to around the 10% mark.. you may as well ask, whats more accurate, the sign saying 10miles to Town X, or your satnav saying 10.2miles to Town X

Ive always found the sound 10% difference between 3 different GPS systems and my car.

However, the same GPS systems in a 6 Series BMW were giving exactly the same readings as the car speedo. I came to the conclusion that its down to the quality of the car but in general the GPS is more accurate.

FWIW when driving on the motorway I tend to use the GPS for speed when travelling through speed camera zones and not the car and so far (*touch wood*) Ive not been flashed.

Next time I see a mobile camera crew down near where I live and I have my GPS with my Ill stop and ask if they could tell me how fast I was going to see whats more accurate, car speedo, gps, or them

GPS only accurate on a perfectly flat and level piece of road, if your travelling at a constant speed according to the guys over at PGPSW and YourNav.

actually military GPS chips are incredibly precise.  Its just public GPS systems that are limited in how accurate theyre allowed to be (to stop them being used as guidance systems).  Its an artificial limit put into them and not a real physical one.

Many GPS systems get around that by averaging a couple of signals.  Old Tom toms also had momentum sensors that corrected speed even further so you could navigate in tunnels (that got pulled fairly recently since people didnt use it).

Your GPS may be a little out but nowhere near what your speedo is.

A polices speed trap camera is going to be more accurate tho as its just a timer counting how long a signal takes to bounce off your car twice.


speed camera > GPS > speedo > hand out the sun roof.

Surely hand out of the sunroof measuring wind speed on a still day would be more accurate still?

the seat of the pants speedo is most accurate