I’ve noticed that in your latest meshes the symmetry plane is not correctly positioned:
Figure 1: Trimmed End Plates Study mesh
I personally prefer to use a whole car and let the background mesh split the geometry. Having a full car will prevent the mesh from trying to slip through any gaps in between the plane of symmetry and the vehicle.
Figure 2: Copy of R5 (SideSrirts_Morgans_Mesh)
Also, I would increase the volume mesh in the x direction; add about two extra car lengths at the front, and 6 to the back. You can have have a bigger cell size further away from the car in order to limit the cell count and there the gradients should be lower.
Other than that, if you follow the FSAE Workshop, you should be OK.