This tutorial is for a freeform generalized car model. It can be adapted towards specific models by using plan drawings in orthographic view and adding extra edge loops as needed. (NB - You'll need to enable Advanced menus for this tut - via Edit | Prefs)
Start with a basic cube.
Extrude outwards the hood and trunk volumes.
(Bonnet & boot for those more used to the U.K. terminology.)
Extrude upward the cab volume.
Scale car, move edges, etc. in order to make proportions more car-like.
Note window angles, roof line, body lines, cab profile, etc.
In addition to being shorter than the body, the roof also tends to be less wide as well.
Select edge ring (default hotkey "g") and connect (default hotkey "c") to create centerline.
Move centerline edges as necessary to define shape a little better.
Note that on most cars, the front and rear windshields have some curvature.
Add these edges to help define the lower body.
Can be done rapidly by selecting appropriate edge ring, hitting "3", and then connecting.
Add these edge loops to help start the front wheel wells.
I almost forgot. These edges must be added as well before doing making the wheel wells.
Don't forget to adjust where needed to maintain proper curvature on cab windshields.
Select these faces and dissolve.
Connect vertices on front and back corners to create diagonal edges.
(A second diagonal edge is in the front of the wheel well area.)
Making the edges may take more than one step.
Connect the front and back diagonals created previously.
Scale edge Radial | radial Z | Scale to point. Select same point as illustrated. Scale to 0%.
(Holding shift will make snapping to 0% easy.)
This is faster than two separate vertice flatten operations.
Create edges inside wheel well (connect, cut, etc.) so that existing edge loops maintain continuity.
Create these "control" edge loops on the cab area of the car.
Using slide twice makes placement easy.
First slide relative +/- 100%, then without deselecting slide distance desired amount.
Create another set of control loops as in the illustration.
Create and place an edge loop as illustrated.
Select edges and slide back as shown in illustration.
Select these faces and repeat the same steps used earlier to make the front wheel well.
Select faces as pictured and scale along Z axis.
(Faces on front inside of wheel wells are also selected but not visible in picture.)
This shapes the wheel openings.
Select these edges and move on Y axis to further adjust wheel well shape.
Loop cut along the centerline. Select the unfinished half and delete it.
Select resulting centerline face, then switch to verts. Flatten verts along x-axis.
(This step works with my personal workflow as I've had problems with the virtual mirror.
It ensures centerline verts actually lie along the centerline when using a RMB reference.
If you find the virtual mirror to work better, you can adapt your workflow to use it.)
Select centerline face again, mirror.
Of course you would think that it would be done in the previous step.
As the smoothed preview here shows, this is definitely not the case.
(This is true if you plan on smoothing it later.)
Right now it would be a bit "nerfy" if smoothed.
Thus a couple more steps to go.
Select all the faces inside the wheel wells and extrude region normal.
Create these controlling edge loops.
Select these faces. Extrude region 0 units. (Use tab entry and enter 0, or use shift to constrain.)
Without deselecting faces from before, hit "l" (L, not 1). This will select the edge loops that go around the edges. Slide inward.
Select the faces on the sides as shown. Extrude region outwards a small amount.
Select edges shown. Slide so that edges move inward on the side.
Create some more "control" edge loops such as the ones highlighted here.
Let's see how it looks with basic wheels, a few more minor tweaks, and in smoothed preview.
Pretty good, isn't it? You can either use the model as it is for low poly applications, smooth a few times, or use it as the base mesh for a more detailed model. (Maybe the final result here will be the start for a later tutorial on modeling detailed cars in Wings 3D.)
After modeling cars this way a few times, it should become obvious as to how the mesh can be changed for different models.
Wings 3D: Tutorials: UV mapping a car
Wings 3D: Tutorials: Making a detailed car