Bull crap, if they have another vehicle that is going the speed limit they could determine it, but the human eye cannot tell the diference between 55 and 60mpg let alone 55 and 57. I would still fight that crap and chances are with most cops when you let them know they will be going to court they are going to save themselves the trouble and write you a warning. A friend of mine got a ticket from a park hills cop while driving on a latonia road doing 37mph, he had rather loud exhaust and the cop wrote him a ticket for doing 50 in a 35, he was too scared to fight it.
Alan