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MonGnoM You need to make the car look alive, put simply. You do this by taking all the energy from the background and focusing onto the car. This only really works when you have a very precise idea of what angle you want to shoot at. For example, the photo below took several attempts to nail.
I tried different shutter speeds, but I settled for a low one, because it directed more movement onto the car. A lot of people don't bother adjusting depth of field, so that makes everything look way to clear and plastic. Only ever set this high if you want to get a wide still view such as this:
Things such as lighting and filters have a huge effect on getting the right feel to your photo, so if you have the right sense of motion and life in your shot but everything looks too bright and over-exposed, try and tone down the lighting and mess around with filters until you achieve the desired effect.
I use a lot of the Speedhunters guides, they are quite good because they focus solely on cars:
http://www.speedhunters.com/tag/photography-guide/