The issue with car speedometers is that there is no negative tolerance in the indication, therefore the car makers have to adjust to some small percentage higher. This is to allow for inflation levels (temperature or maintenance) or the car being fitted with non-OEM tyres
If you fire up Waze, you can compare the clock with the GPS and that will allow you to calibrate your car reasonably well in reality. Mine is 2mph over at 50 for example. If I set my cruise at 75mph, then I can be pretty sure I won't fire a camera on the motorway (or at least it hasn't happened yet touch wood). I could probably get away with 80 on the dial, but why bother. It won't make much difference to my arrival time, especially if I stop somewhere for a coffee.
I had a chat with some speed coppers at Goodwood once, and they said there is a +10% discretion (otherwise they would be overwhelmed with business, and lose the trust of the public). On top of that, different counties apply a bit extra too. West Sussex is 10% + 3, and Hampshire is 10% + 2 for example. However, they are perfectly within their rights to book you at lower if you're driving like a wally. So it is discretion, not a rule.
I share Tony's concern about cruise control. But then I got adaptive cruise on my last car, and I wouldn't look back. I'm afraid on long motorway runs, I drive using the stick on the column rather than the pedals, but no, it's not a good idea to put your feet up on the dash