If the speedo isn't calibrated then it could be off. Best way to tell is jack up the rear of the car and count how many times the driveshaft spins for 1 rotation of the wheel. Should be fairly easy to tell if it turns 3 3/4 times or 3 1/4 times.
On my old 96 I did 3.73's without calibrating the speedo and it was about 10-15 mph off.