One thing to be careful about are the geometry intersections. Â These are to be avoided because they tend to cause problems. To minimise problems, some of the topology might have to be rearranged to make the separate pieces match at a vertex level. This way, the smoothed meshes will intersect without causing rendering problems.
Before applying smooth, some of the edges of the model have to be "strengthened" by adding some parallel cuts to them. Â This can be done by using either the Split Polygon Tool, or Select Ringand Split Edge Ring.
For the wheel cut-out, a small extrusion will make the area more believable as most cars have this detail.
One thing to be aware of is that, for the seams between parts to look good, the topologies have to be similar and the meshes should have matching vertices on the borders. Â This will ensure the objects will have identical borders even when smoothed.