Constant speed horizontal axis wind turbines operate at power coefficients different than its maximum due to operation at different tip speed ratios (TSRs). Whereas rotational speed and the site wind speed distribution determine the TSR range of the operation, the pitch angle is significant for the aerodynamic performance of the wind turbine. In this paper, a blade element momentum approach is used to study the power production effects of pitch angle and rotational speed of NREL Phase VI wind turbine at different site wind distributions (characterized by Weibull distributions). It is shown that the best combination of rotational speed and pitch angle always occurs at the same pitch angle, but for different rotational speeds (other than the best one) the best pitch angle may vary. The best rotational speed is found to be dependent on both form and scale factors of Weibull wind speed distribution. Improvements in turbine pitch angle and rotational speed are found to increase generated energy up to four times, but the increasing factor is highly dependent on wind speed distribution: For the studied turbine, high-wind-speeds sites can benefit more from the adjustment of these parameters than low-speed ones.