To significantly enhance the economic performance of electric vehicles operating in platoons and mitigate the rate of power battery capacity degradation, a comprehensive study on driving speed planning was undertaken specifically for a given electric truck platoon. Firstly, a single vehicle energy consumption model for electric trucks was established; then, leveraging this foundational single-vehicle model, a strategy incorporating a fixed headway following distance, along with aerodynamic drag coefficients derived through precise fitting for each vehicle within the platoon at varying distances, facilitated the development of an energy consumption model tailored specifically for the platoon operation. Subsequently, the alternating direction method of multipliers (ADMM) was used to optimize the speed of the vehicle platoon, aiming to minimize energy consumption while adhering to constraints such as travel time and road speed limits. Ultimately, the influence of the planned speed on both the average energy consumption of the platoon and the battery capacity decay was simulated and analyzed. Research has shown that compared to the speed of adaptive cruise control and the speed planned by the quadratic programming method based on road slope acceleration in the platoon, driving at the speed planned by ADMM in the platoon the electric vehicle can save energy by 9.30% and 7.76%, respectively; and with the increase of driving mileage, the energy-saving effect of driving at the speed planned by ADMM becomes more and more obvious; After one year of operation in the vehicle platoon, the battery capacity decay would be decreased by 9.69% and 2.54%, indicating that driving at the speed planned by ADMM can slow down the aging rate of the battery, and over time, the difference in battery capacity decay between these three driving speeds becomes increasingly significant.