Here is a screen shot of my Civic, no mods. I used the settings 120 and 44 because it makes the flow look how I think it should for my car. Notice the flow becomes detached at the top of my rear window, and then skips off the back of my trunk lid.
This is what I am 95% sure is what is happening, so these settings approximate what air really does. I recommend you use these settings when you want to have an idea of how your vehicle will behave as well.
My image size is 600 X 300.
You'll notice as well that I have the body of the car at its apprx height off the ground with no wheels. This simulates in 2D, so keeping the wheels on will only lets you see how the air would flow under your car if it had 5 foot wide tires like a steam roller. You really want to depict the centerline of your car, so that's why I have the little axle bumps there.
The interaction of the air going under the car with the air going over the top is important. As well, the air going over the top behaves very differently depending on where it is in relationship to the ground. The solid black line across the bottom is my ground plane which I added on there to act as the "street". Not sure if the program recognizes the bottom of the screen as a "ground" so that's why I do it.
I also put the car forward in the frame to see what the air is doing in the wake, I really don't care about the front. This is what I think you should try and replicate if you want a reasonably meaningful look at how your project will affect airflow.
Here is a stock 4dr Echo with the same "treatment" and settings.
