Wheel wells are modelled quite simple. First of all, select vertices on the
wheel wells boundary, as it's shown on the image:
I've drawn a blue line over the vertices that should be selected. Note, that each selected vertex must be on an "open edge" (it should not be inside any polygon - only on boundary). Switch to Selected and Screen-X modes. Pick a Modify\Submesh\Extrude tool. Then extrude selected vertices in Front aproximately to a depth of wheel well:
Drag Pivot helper in front view aproximately to the depth of wheel well (where
extruded vertices are located). Pick Modify\Scale tool, click and drag
mouse down in Front view several times. You will notice that extruded vertices
will lineup into straight vertical line over pivot helper. Like on the image
Deselect vertices and switch selected mode off.
In Perspective view, get rid of way too unnecessary detailed wheel well
vertices by merging some neighbour pairs. Image below shows where detalisation
can be reduced:
Make sure to leave bottom vertices as is (I haven't merged them with any others). In general, amount of innerside vertices can be reduced to four in each wheel well: a pair of bottom vertices and a pair of vertices made from all other. You should keep in mind that wheel well is always dark and all the time there is a wheel there - you will not see that much details inside.
The last step is to fill gap with a polygon(s) in each wheel well. I prefer to
fill it with single polygon. Pick a Create\Polygon\Single, make sure to
change from Triangles or Quads to Polygons inside options
box of Create\Polygon tool. Now adjust Perspective view to see all
innerside vertices of wheel well. Create a polygon by clicking sequentially
on vertices and right-click when done:
Repeat the same with another wheel well.
Deselect all vertices. Pick a Select\Open Edges tool and right-click on
any of "open edges" vertex in the bottom. The tool will select all opened edges
in the mesh:
Invert selection with Select\Invert tool and hide all selected vertices. You will come up to something like on the image below:
Since we need to model bottom of the car, hide all unnecessary vertices and leave visible vertices that belong to bottom only and move pivot helper to height of bottom (in side view) since we shall create some polygons and vertices now:
Image below shows how polygons were created. First of all, in bottom view I've painted an area that should be filled with polygons. Then polygons were laid. Notice, that you should not use "polygons" type where result surface is not flat (I have useed several adjacent triangles and only where surface is nearly-flat, polygons were used):
The result of this lesson is in Files\les3_06.z3d file.