You have this strange vertical line on your door where the paint seems to get a different shade or reflection index. And on your render of the top of the car I can see a similar thing on the roof.
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Yes, I noticed that when the pictures were done rendering. I don't know where it's coming from. At first I thought that there was a problem with the mesh (i.e. not connected vertices) but the structure is fine. I haven't investigated further, but it probably has to do someting with the material itself. For the sparkling effect I applied a voronoi noise which is scaled up.
Looks good, but I think the shadow under the car could be a bit sharper. Less global illumination and possibly more directional light from the sun should do the trick.
This is because there are no further lights than the HDRI background image present at the moment. The correct setup with the background picture an proper lighting will be the next step. I am testing various HDRIs for the correct color and reflectiveness of the cars' painting, since the appearence is different with each scenery. The rendering you are referring to is the "best" compromise (without putting any further effort) yet.