Regarding the speedo - mine appears to over-read by about 2-3 MPH - that's according to my Dash Cam GPS. If I'm in a 30 zone, with cruise on, I generally set the cruise to 33 MPH and the speed limit signs on the road tends to flash green (30 MPH) when I pass.
I think it may be over slightly, as a precaution to make sure you don't go over the speed limit.
If I set it at 30, my Dash Cam GPS reads 28 MPH.
Citroen state the following regarding differences:
The user may see a different vehicle speed displayed on the instrument panel and on a driving accessory (GPS - speed camera alert...).
The driving accessories (GPS, speed camera alert...) work by calculating the distance between the GPS receiver and several satellites. The accuracy depends on the quality of the signal which may be interrupted or disrupted (interference, radio interference, satellite malfunction...).
On the other hand, the vehicle’s ABS system determines the vehicle speed accurately by calculating the arithmetic mean of the information transmitted by the wheel sensors.
So Citroen assert that the vehicle wheel speed sensors are more accurate - but I'm yet to be convinced, unless the road speed indicator units that work by radar are also faulty.....
According to Auto Express who did some checks on speed cameras, they were testing vehicle accuracy speedo accuracy and stated:
Car speedometers are not allowed to ‘under-read’ – they can’t tell you you’re going more slowly than you really are – but they are allowed to over-read by up to 10 per cent plus 6.25mph. So they could read 50.25mph at 40mph.
