It depends on how much you want the simulation to be closer to the real situation.
If you only interesting in one dimension motion and want to know how position/velocity changes with time.
You can treat the car as a point. However, the drag will depends on the cross section of the car.

If you want to simulated different  weight between front/rear tires, you need to calculate different normal forces when car is accelerated or de-accelerated (Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=452.0]Normal forces on front and rear tires when car is accelerated[/url]!). 

You need to re-calculate it if the car is moving up or going down a hill.

Write down detail description of your model (with equations) and I will check it for you!