No speed gun is going to have ANY ERROR, it works on the DOPPLER principal and they are tested before use EACH DAY, so that they are accurate, a not also has to be made in the PC's notebook.
Newer versions of the speed gun are merely pointed at a REFFERENCE PLATE on a wall in the Police station yard, this has been measured by a SURVEYOR and a certificate of distance issued, from then point at where to Pc holds the gun and the plate he points it at, the gun then calibrates itself.
Detection tolerances vary and is up to each individual force to set.
All those that say they know are guessing or a PC as they do NOT give out this info as it defeats the purpose!!
It can be assumed however that it is the advised limit of 10% + 3mph, as a minimum.
The ones with cameras at each end are called SPECS, which measure your average speed through a particular place such as roadworks etc, changing lanes does NOT have any effect despite what some say.
These will become more widespread as they read plates and we can all be watched more closely, looking for evidence of criminals movement, hence why the gov are considering driving bans for offences not involving the road!!
There is a smaller version of the speed camera appearing, which is about 1/2 the size of a shoe box, these do NOT have to be painted etc.
tabetha