The car manufacturers speedos have an accuracy of 10% so they should not fine you if you are doing 65.9 km/hr.
To be honest, I think everyone has heard this. I'm not going to dispute it but I have never seen anything that (IMO) absolutely confirms this nor am I really interested in doing so.
The variation is going to be consistent ie if your speedo is reading 60km/hr one day and the actual speed is 65, then the next day if your speedo is reading 60km/hr your actual speed will still be 65. Not 60, 55, 70 whatever. The one very insignificant difference could be due to temperature as this does affect the way components behave mechanically however that would be so small as to discount it.
If your speedo behaves contrary to the above statement then you have a defective instrument. I would stand by this even on the old speedos that we find in our classics.
So if you are aware that your speedo over / under reads then YOU are responsible for adjusting your driving to suit.
So then we can throw in the argument about the accuracy of the various detection devices in use... well, again these are, under normal circumstances going to be pretty darn close to the mark.
They do make allowances and that is why you don't (typically) get done for speeding 5k's over the limit.
I used to drive like there was no tomorrow at times, certainly, and when I got caught, I ACCEPTED that I was doing the wrong thing and paid the fine. I even had a suspension once for too many points. That was all my own doing and so be it.
I am happy to call the speed cameras revenue raisers as, once again I will say, they are a voluntary tax. If something is wrong with the gear or an officer has made a mistake then absolutely contest it but if you are doing all the right things, observing the law then you should be just fine. Where the revenue gets sent to, well that is up to our politicians and guess what...? We, the voting populatoin put them in office.
Just my humble opinion...