I come from a comic/traditional background so its mostly stuff I've picked up from that area.
If I had to break it down I'd say I started with a basic vector shape of the car with the changes I had make and then applied highlights and shadows as if it were a comic rather then going for strict realism (but that's not to say I totally ignored my starter image). After I was happy with the shape of the car and everything looked as if it fitted and lighted correctly in photoshop. I then went back to my vector pic and went about adding the race markings this actually took quite long as I originally wanted to go with the Denso Sard colours but I found the NFS markings really made and impact. Then I just spent a few hours just adjusting little things just for the sake of experimenting as I'm still fairly new to vector art.