Creating wheels gaps and cars' bottom is, as a rule, the final step in creating the cars' body. Of cause, you can also create interior, mirrors and wheels after it, but currently, with this step we create the closed volume of the car.
       Switch to front view and place axis center in the depth of these gaps:

       Disable background images in the views by unchecking Custom...>Show Image option in Views' popup menu.
       Then hide all unnecessary vertices and switch to side view:

       Create the following faces in the gap:

       Then, using the Create\Faces\Fan tool, fill with faces two holes:

... and the same way the second hole, and get the following:

       Finally, move depth vertices aside to make the gap bigger:


       To create cars' bottom you have to hide all the vertices, but the lowest. It will look like this:

       Notice, that I placed axis center on the level of the cars' bottom (not higher, not lower).
       The following image illustrates, how to fill with faces the bottom of the car. You can do it in the Top view with Create\Faces\Fan tool.

       As a result we got the cars' volume:

This file is HERE.

       You can also create mirrors here. As for the wheels - it's a topic for the whole chapter and will be described later.