When you are a "mature" driver, it makes very little difference, the same as NCB, I often have several cars and as a result they don't all have max NCB, so i play around to see what the best options are. Many a time the same car "for a person of my age" the policy difference between 0 NCB and 9...
All car manufactures for as long as i can remember have always made sure the speedo is 5-10% under the actual speed to eusure they can't get later sued later, can you imaging if it was later found they were at fault for many many speeding conviction, incorrectly banned drivers claiming loss or...