RPM,
I can understand getting one speeding ticket thinking you were on the speed limit. Getting 4 is a bit much.
If your speedometer isn't calibrated, and almost all are not, here is an easy way to do it. All you need is a stop watch, a calculator and milemarkers on the highway.
Time one measured mile, mile marker to mile marker. Take the time and divide 5280 by the time. Take the resulting number and divide by 1.47. The number you get after that is your actual speed. Never never take your speedometer as gospel, even a digital one. Check it and be sure.