In recent years, spip owners have been very much interested in the method of achieving the more economical operation of their ships than ever. This paper states the service speed, which cannot be disregarded in trying to solve the above matter. The service speed is for tankers on the route between Persian Gulf and Japan, and the method of presuming the service speed is mentioned here under. First, each element such as fouling, aging, current and wind frequency is extracted from the ships logbook data and the wave spectrum equivalent to that at the actual navigating conditions is predected from loss of speed of actual ship and theoritical added resistance due to wave. Further, introduce fouling area into the concept of effective depth and nondimensional added resistance values due to fouling are within some limit with no relation to ship's size. Aging affect docking interval and we propose that equivalent sand roughness which was estimated from analysis of each ship's added resistance is applied to calculated added resistance due to aging. At last, each calculated element is assembled and the service speed on each voyage of a certain ship is figured out by presumption. This method of presuming the service speed is proven effective by means of comparing it with the actual data.