It is vital that the artist checks the polygons he just created as often as possible, and from as many angles as possible. Checking will probably take more time for each new part/polygon than actually creating it, but this is necessary to ensure the quality of the work.
By extruding, cutting and moving vertices around, this is how the side looks:
The basic shape can be obtained by extruding the border edges of this geometry. Â Notice I haven't extruded the bottom part as it is not necessary to have that area high-res.
Next is the front air intake, the smaller front side intakes and the basic shape of the headlight:
I then cut the shape of the glass roof from the main body geometry. Â I will use the extracted faces as a base for the roof. To get a better view of the entire car I duplicate the side as an instance and scale it by -1 on the X-axis.
For the security arch of the glass roof, make a copy of the glass roof and then cut a shape using the Split Polygon Tool, which will then be extruded to get the desired shape.
The basic shape might be done now, but the silhouette is not, as the car looks "square" when viewed from the front, back and top views. This is because there is no direct reference for these orthographic views.
This problem can be corrected by using a latticemodifier on the body to modify its general shape. This method is good when the result is "square" and will add a new dynamic look to an otherwise dull-looking model. The command can be found under Create Deformers > Lattice. I used these settings but the different parameters can be changed to better suit each vehicle: