C program to design a car?

Proper design softwares takes alot of skills, you might want to star with something like Blander, Sketchup possibly 3DsMax.

I reccommend Google Sketchup.

CAD / 3D software's for car design is used for:

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- They model 3D cars to give a realistic picture of how the car will look.

- To test the car ( Aerodynamics, Cooling, Power, Etc... )

- To make 3D models which other machines might cut out a clay model from.