There is no rule, you can get booked at 1mph over the speed limit. Thing is, every speedometer measures at slightly different rates, so they seem to allow a little leeway..ie..the 10% 'rule'...however, your 77mph might be their 75mph, in which case, you're stuffed if they pull you over. Hope that makes sense!
If you drive quick enough you'll outspeed the police anyway.... Serious point, it's nothing more than a political points winner from a failing government and will be binned when all the safety do gooders submit their objections. I reckon as many crashes are caused by drivers spending so long looking at their speedo they forget what's in front of them as there are from drivers actually concentrating on the road but speeding....
