If your speedo is reading LESS than the actual speed of the car (how did you check it? from a satnav readout I guess) then that can be a serious error. It means you can be exceeding the speed limit by 10kph and think you are within the limit. As you can see from the quote below, your speedo should be designed to over-read. On my jazz the speedo is about 2mph optimistic, which means at an indicated 60mph my satnav tells me i'm doing about 58mph. I would check that your tyres are within the diameter allowed, if they have been modified and are bigger than the tyres on a 'standard' Jazz, then your speedo will read under (the wheels are going around slower than they should be for any given speed).
http://www.speed-trap.co.uk/Accused_Home/Rules_useage/The_Law.htmVehicle construction and use regulations require a vehicle speedometer accuracy to be in the range of
-0->+10%.
The implications are that it must never under-read - for obvious reasons - but may over-read. As the cost of manufacturing a speedometer with -0% error would be very costly they all over-read by a few percent without exception. Even if speed is measured correctly the display may not be accurate, so a speedometer error is allowed. Because of this, the Association of Chief Police Officers (ACPO) have an official formula for calculating a speeding offence. It allows a leeway of 10% plus 2mph. In reality, most speed traps are triggered at higher speeds than this because if they were set bang-on those guidelines, the sheer amount of paperwork generated would overrun the police speeding departments.
