Quote Originally Posted by Des Parrot View Post
I cannot say if this is still the case but it was: +10% +3mph, which was in place for analogue instruments, still used on older vehicles, some commercial vehicles and motorcycles.

30+3+3 =36mph
40+4+3 =47mph
58 in 50, 69 in 60, 80 in 70
I guess now 25 in a 20.

I stress that this may have changed but I haven’t seen or read anything.
It's 10% plus 2 MPH. That is to say that they give you 1 MPH and take the photo at 2. I was done on the M4 in a 50 limit at 57, ie, 50 +5 +2.
however beware as this is a government guideline and certain constabularies ignore it to a greater or lesser degree. (I believe Nottingham is one) If you go on line you can find a list of forces that use the government guideline. SW Police do but Gwent police failed to answer when asked in a freedom of Information request.

Additional to this is the fact that all speedos are in error by at least 2 MPH in favour of the driver avoiding a speeding penalty. My own car speedo is 2 MPH fast up to about 35 and then 3 MPH. In theory it means you could travel on the motorway with your speedo reading 80 and not be caught. however I wouldn't chance it.
Because of the points I have been extra careful for the last 3 years but usually add 10% to the limit (Conditions permitting) and I've never been caught. The plus 1 and the speedo error is my buffer zone.
An easy way to check the error on your speedo is put in a route to google maps and set your cruise control. Then compare the speed on the satnav to your speedo reading. (you may have to go on a 40 or 50 MPH road and set the speedo at 43/54 )