yo udon't jsut get off a ticket for your speedo being off, its something that if yo uahve a GOOD record they might take into consideration and reduce it to a lower speedin charge, like 1-9mph over.
also just b/c the cop doesn't show doesn't mean its thrown out, usually you just have to come back again teh next time hes in court as well.
i'm pretty sure too if he didn't write you up for wreckless, only regular speeding then it only will be a regular speeding 4pt charge.