It is important to reduce aerodynamic drag for reducing fuel consumption. Conventionally reduction of aerodynamic drag has been carried out by shape optimization of each part of a vehicle based on the investigations of the time-averaged flows around the vehicle. However, the general tendency of drag reduction has been saturated recently and it is required to develop a new flow-control technique to achieve further reduction in aerodynamic drag. We therefore focus on the unsteadiness of the flow around a vehicle to achieve it because the aerodynamic drag of a vehicle fluctuates over time due to repetitions of generation, growth, merging and disappearance of various sizes of vortices around it. These vortices are formed by flow separations, for which the longitudinal coherent vortices inside turbulent boundary layers on vehicle surfaces are presumably playing an important role. However, there have been few studies on these vortices due to the difficulty in performing such a high-resolution flow simulation that is able to capture these small structures. In fact, the size of these vortices in the boundary layers on vehicle surfaces is estimated to be as small as 0.4 mm when the vehicle is running at 100 km/h. In the present paper, we carried out wall-resolving LES (Large Eddy Simulation) for 1/4-scale simplified 2-box car models. The Reynolds number based on the vehicle speed and the representative length of 1.06 m is set to about 0.71 million. By using K computer, which accommodates approximately 80,000 CPUs, we have realized large-scale flow computations by using approximately 2.9 billion grids. At first, we investigated appropriate grid resolution by considering the size of the longitudinal coherent vortices in the boundary layers and we have confirmed the importance to accurately resolve turbulent boundary layers in the under-floor flow of a car model. Next, we have identified critical differences in the vortical structures behind car models that have high and low aerodynamic drag. Especially, we have found that the hone-shaped vortical structures are formed behind the car model with a drastically worsened aerodynamic drag, and proposed a formation process of such vortical structures. This new finding is expected to help us achieve further reduction in aerodynamic drag of 2-box vehicles. In addition, we have found that the under-floor flow plays an important role in the above-mentioned formation process and it is essentially determined by the under-floor shape of a car model and is little affected by the flows over the roof, C pillar, or side surfaces of a car model.