ok, how old are you, 12?
the thing is, even if there is a law that says that this speed measurement was illegal, dont fuck with cops. in german we say (translated directly) that they "sit at the longer lever", meaning that they can bust you for everything if they dont like you or think that they dont respect you.
idk about the us, but just for car controls, they could ask for first-aid kit, safety vests, how to access the spare wheel, pressure in tires, profile depth of the tires, etc etc etc until they find something not correct and give you a ticket for it.
bottom line: stay calm and accept the punishment, especially in such obvious cases. i hate people who can only blame cops, teachers, politicians and parents for their sorry excuse for a life.