In the course of investigation into the operating economics of transport aircraft, the cruising speed arises as a fundamental variable and a rapid method of correction of speed for variation of power, height and weight is frequently required. Other problems present themselves, such as the estimate of cruising speeds in the course of a new type design study, or the correction of a “one–point” published performance of a new aircraft to a standard operational condition differing in specified weight, altitude or power output for cruising.In this note no account will be taken of the block speed correction arising because of the modification of true air speed by the various factors, such as wind, climb and glide which distort the optimum flight path. The principle of the method here suggested involves the unique relationship of p, the ratio of thrust power required at any speed (V) to the thrust power at the minimum drag speed (u) at the same altitude, and n the wellknown speed ratio V/u.