A simulation framework for vehicle aerodynamics using up to 10 billion fully unstructured cells has been developed on a world-fastest class supercomputer, called the K computer, in Kobe, Japan. The simulation software FrontFlow/red-Aero was fully optimized on the K computer to utilize up to 10,000 processors with tens of thousands of cores. A hybrid parallelization method using MPI among processors and OpenMP among cores inside each processor was adopted. The code was specially tuned for unsteady aerodynamic simulation including large-eddy simulation, and low Mach number approximation was adopted to avoid excessive iterations usually required for the fully incompressible algorithm.The automated mesh refining system was developed to generate unstructured meshes of up to 10 billion cells. In the system, users only generate unstructured meshes in the order of tens of millions of cells directly using commercial preprocessing software. Then the original elements are refined in the preconditioning process using the K computer, and billions of cells are automatically generated. In the same way, the CFD results of the billions of elements are mapped on the original coarse grid points during post processing, and the users can transport the data from the K computer without any difficulty visualizing the results using conventional post-processing software.The simulation framework was validated on vehicles with different rear slant angles, and dependence of the aerodynamic forces on the unstructured-mesh resolution as well as the grid topology near the body surface was discussed. It was demonstrated that the simulation of the multi-billion cell mesh on the K computer agreed with wind-tunnel measurements within a small percentage of error, and validity of the fully unstructured finite volume method using multi-billion cells was confirmed.