A speckle velocimeter exploiting the effects of external optical feedback in a semiconductor laser is analysed. The laser diode is used both as the light source and as the detector making the system self-detecting and self-aligning with a single optical axis. The velocimeter employs an autocorrelation method to analyse the detected self-mixing speckle signal. The linear relation between the velocity and the reciprocal of the autocorrelation time of fluctuations in the amplitude of the speckle signal allows the determination of the velocity. Effects of various parameters of the measurement system are analysed and the dependences of the reproducibility and accuracy of the measurements on these parameters are discussed in order to achieve optimization of the system. The optimized system has a measurement error of less than 1.4% within the range of calibration velocities and less than 2.7% outside this range.