The results of converting the air-jet engine speed characteristics into a form that allows an unambiguous estimation of their efficiency are presented. It is shown that the most convenient parameter for characterizing the efficiency of air-jet engines is the ratio of the aircraft flight speed V to the specific fuel consumption (SFC), since this ratio represents specific thrust work per unit fuel mass, and it is proportional to the overall efficiency of the propulsion. The speed characteristics of the D-36 engine have been obtained in the converted form, demonstrating that its efficiency increases with flight speed despite a rise in specific fuel consumption. Using the proposed parameter, a comparison of the efficiency of propulsions in cruise modes has been done, confirming the necessity of accounting for the flight speed. The role of this complex in the Breguet Range Equation for calculating the theoretical range of an aircraft is also demonstrated.