Accurately determining an aircraft's flight speed is crucial for optimizing airline performance, as it directly impacts factors such as fuel consumption and emissions. Flying at speeds higher than what is recommended by the manufacturer can result in increased fuel burn. However, flying at slower speeds may lead to longer flight times and competitive disadvantages for airlines as passengers typically prefer shorter travel times. This study empirically investigates the driving forces in the decision-making process of airlines when setting flight speeds to reduce costs while maintaining the quality of service provided to customers. We develop econometric models of planned flight cruise speed and actual mean flight speed. We analyze a vast amount of data, comprising millions of domestic flights within Brazil. Our results allow for policy recommendations that identify opportunities for improvements in airline flight operations optimization, with implications for the environmental footprint of commercial aviation.