How to calibrate a speedometer on a 1991 nissan hatchback pulsar?

Most speedometer are optimistic about 5 to 10% so your car is not very far out of calibration. Old Spinning cable type spedo's have a magnet that spins inside a bell that is spring returned hooked to a needle. Even modern electronic instrument clusters have to be optimistic so people drive safely and cannot say there spedo showed slower than they were actually were traveling. Size of tires low profile could make your wheels spin faster and make readings even more inaccurate. There may be an old fashioned Spedometer shop still working in your area. Good Luck