I'm running a simulation of a land vehicle, and I'v added a ground plane but I have found some problems while adjusting the computational domain settings, here are my basic settings for the Solidworks 2010 flow simulation:

-The vehicle is set on a big solid rectagle, which is the 'ground'.

-The computation domain is set to .001 m below the  surface of the 'ground'.

-The top surface of the 'ground' has the boundary conditions of "real wall" and is set to a velocity that the air is moving. 
-The 'ground' settings are: 20 m/s along the Z axis, Surface roughness of 50 Micrometers

the study is  set to an external study, excluding internal volumes.

-The goal for the study is Force in vehicles moving direction axis Z

Everything else is pretty much clear, but the problem comes with the computational domain and it's position.

Results vary depending on how much Computational domain is set below the ground, and if its set to 0 or above the ground, computing is not possible due to the missing boundary wall conditions.

Also when inserting a cut plot, its clearly visible that the front area of the ground also interferes with the flow, velocity is decreased in the very front area, why is that?

Can you explain what should be done to make ground act as a real ground without messing the results ?

Below is the picture of the simulation, vehicle is removed from the picture.