Nowadays, ships' energy efficiency plays a key function in reducing fuel consumption and in the safety of the maritime environment. EEOI is an energy efficiency monitoring indicator for existing ships. This research aimed to enhance the container ships' energy efficiency and environmental sustainability by speed management during ship operation. Speed management strategy as an energy efficiency improving measure, could make more sense while applied on container ships due to the higher speeds of this ship type. This paper also outlines the calculation procedures for ship-based GHG emissions. In this paper, a 700 TEU container ship has been selected as a test case. Actual statistical operational data of the candidate vessel have been reviewed for three consecutive years and specifically analyzed to examine the impact of reduced speed on EEOI, consequently on ship's emissions, and fuel consumption. This study showed enthusiastic outcomes relating to emissions reduction and EEOI improving as well.