The speed log is a crucial source for data in the performance calculation.
The accuracy of the sensor depends on the calibration and on the manufacture.
An offset between speed log readings and the actual speed will indicate the need for calibration.
With regard to accuracy, various manufactures of speed logs give various information.
Once calibrated the speed log measurements may have an accuracy of 0.1%, Griffiths et al. (1998), but series of factors influences the measurements.