This paper presents an energy-efficient velocity control technique for a truck driving behind a slow moving truck. An observer is designed to estimate the acceleration capability of the leading vehicle, which is then used to predict the velocity and time trajectory of the leading vehicle. This information, together with information of road topography, is used by the ego vehicle to optimally plan its velocity along a look-ahead horizon. The optimal planning is achieved by nonlinear model predictive control, where constraints are set to keep a safe distance to the leading vehicle and arrive at the destination within a given time. The designed controller is tested on various driving cycles. The proposed technique is also tested on a traffic light scenario, where information about the position of the traffic light and timing of its signals is considered to be known. The simulations results show that the proposed technique can save a significant amount of fuel.