Speedometer Calibration Help Needed

Does anyone out here know how I can recalibrate my speedometer? It was reading properly until I changed out my rear tires and now it is 13 -15 MPH off from what speed I am traveling at. Example: If I am traveling at 65 MPH, my Speedometer is reading 50 MPH. I have now got my 5th speeding ticket as a result of improper readings from my Speedometer and I am using my GPS as a temporary Speedometer. I am not sure If I would need to count the gear and reinstall the gear back into the transmission in another position from where I originally set it. Any assistance is greatly appreaciated as I cannot afford another ticket, I do not want to lose my license.