Good Morning Oggie13
Thank you for your post.
The speedometer in the vehicle overstates the speed by approx. 10% to allow for wear, over time, of the tyres and to make sure that the driver of the vehicle cannot accidentally drive over the legal speed limit for the road.
This can be demonstrated by comparing the reading on the instrument display with the speed reading on the Sat Nav as the Sat Nav is calibrated by GPS positioning and the internal pack is calibrated using the wheel speed sensors. You should notice a small difference in the speed readings.
Please note: You should not rely on the Sat Nav reading as a defence for speeding however, in case there is an error with the GPS signal at that time.
I hope this has answered your question, should you require any further assistance please do not hesitate to contact me.
Many Thanks
Dan - Jaguar UK