There has to be a margin of variance due to different car setups tyres wheel size age of the car if its a cable speedo they stretch and become less acurate.
I used to be 10%!!!!!!!
I any case I tell you what gets on my T*TS is when you come up to a varible speed section and you slow too the speed and everyone else zooms past how does that work !!!!
Also when your in the zone they still come past and your doing the limit !!!