My wife is always complaining that I drive the Dyane too fast against the other vehicle she sometimes travels in my working van a Citroen Jumper/Relay. So yesterday on the way to a job I set the GPS unit to read speed and I was shocked to see that when the van speedo said 100kph the GPS ws only reading 85kph. So today I took the Dyane for a 200km drive and took the GPS out of the van to check the Dyane speedo and when the speedo was reading 100kph the GPS was reading 104kph............
The van is as it came out of Mr Citroen factory a few years ago with correct size tyres etc. so a difference of 15kph below the true speeds seems crazy..

If the GPS is correct I will have to drive slightly below the speedo speed especially in town as 50kph on the speedo is 52kph on the GPS.

The long and short is my wife was correct....

You could check them both at the same time just hammer through speed camera .
I'm fairly sure that GPS does not agree with speeds show on your clock.

http://gpssystems.net/accurate-gpsbased-speedometer/ This should help to explain

I have passed many speed signs on the side of the road (the one that gives you nice face and speed if under or sad face and flashing red speed if over) in the Dyane and they are very close to the speed I see on my speedo. There is a new one in a village near me (saw today for first time) and it read 49kph and I looked down and it was roughly just below 50 on speedo. I will check in the van next week what that says.

The GPS is generally correct. My van reads about 5 kph to fast but I kinda like that as I see my 75 kph top speed (when it is only 70).

We have a radar speed indicator coming into our village - I was doing 31 kph on the sole yesterday. But on my road bike I can top it out over 50 before I have to turn.... need more solex power

And when I say GPS I am talking about the one on my road bike not a car unit.

Nothing new here......... wifes are Always correct.....

Even when they are not correct...

The other halfs dad said to me first time i met them - always take the blame, because in the end itll be your fault anyway so save yourself some time.

Im always RIGHT!

- unless She is Righter

There is usually around a 10% optimistic tolerance on speedos so 15kph is a little high but not un-expected.

I've just come round a bit of the M25 at an indicated 70mph on the speedo which was 67 on the tomtom. 30 is spot on, indicated 40 about 38.