mines out by 6 mph.
don't forget its a safety feature, most normal people, (especially before the days of satnav) would be none the wiser. so if your speedo said 70 you though you were doing seventy. but most likely it was 65. it means on those odd occassions you weren't watching your speedo like a hawk and were accidently doing 74 and you went past mr plod or a camera, you didn't get points or a fine as you were still under the speed limit.
i use my satnav speed when travelling through average speed areas as it means i can overtake the normal people while still being at the correct speed.