Mine too, in Colony, KS. 63 in a 55 zone. Turned out my speedometer was reading incorrectly - I was doing 55 when the speedo said 48.
My first as well (late '74 or early '75), doing 74 in a 55. :/
Come to think of it, I actually haven't gotten a speeding ticket since. Though I did slip off on a side road to avoid one once (shortly after the first one)...
I'm afraid mine was rather intentional. Two lane road, hilly country and I was driving an 18-wheeler with heavy traffic. I was trying to gain speed downhill so I wouldn't get too slow going uphill when the highway patrol caught me at the bottom.