If you were doing more than 65 MPH the cop most likely did you a favor because in most areas more than 20 MPH over is considered reckless endangerment and has far higher penalties. In some areas this means an automatic suspension of your license, usually for 90 days.
And yes he probably knew exactly how fast you were going. There is absolutely no requirement to tell a motorist the maximum speed you clock them at, only how fast you are citing them for. It is perfectly acceptable to cite a motorist for a lower speed than they were clocked at.
What it amounts to--
Be honest with yourself. You admit you were speeding. You hint that you were doing substantially in excess of what you were cited for. Pay the ticket.
SaburoS--
Why do you assume that the officer has to be lying if his clocked speed is higher than actual speed? Do you know how lidar/radar units work?
J_A_B