A way to save energy is to reduce the drag of ground vehicles. To do so, the platooning of ground vehicles on top of a road, in particular highways is studied. The numerical simulations are performed in 2D and 3D for up to 10 billions unknowns on 384 cores. The goal is to have trucks autonomously following their leader to form a road train in order to improve traffic flow efficiency and to reduce oil consumption. Thus the distance between trucks is short. For instance a gain of about 40% can be obtained on the drag coefficient of the followers when the distance between trucks is equal to 1.8125 their height (see the figure), that is approximately eight meters. Even the leader has a lower drag coefficient (-10%) as the first follower compresses the flow in its wake. In the figure it is clearly shown that the pressure gradients inside the gap between the vehicles are much lower than in front of the leader.