There are several levels of parented objects.

The main parent is the empty "Main_Control". It is used to move the car and has a shrinkwrap constraint to the floor object.

"Main_Control" z-value must be below the floor object in order for the shrinkwrap to work.

Each wheel has an empty on it's bottom side. These empty work the same way as the "Main_Control".

You can set any object as floor object as you want. Make your floor object the target of these empties' shrinkwrap constraint.

Note: If the floor objects geometry is too bumpy, the shrinkwrap constraints might be buggy.

Note: if you want to export this for use in a 3rd party program you may first need to apply constraints in Blender.

Objects that use constraints are: all suspension springs, suspension housings, suspension pistons, rear axles, drive shafts, front suspensions and steering rod. Also the flamethrower hoses.