Another technique is to use ambient light with a flash during the day. Shoot at 1/60 or slower and pan with the car. Use a fill flash and the strobe will help to freeze the car, but the background still blurs since it is such a slow shutter speed and still exposing the frame. A good idea is to shoot your ambient exposure at about 1 stop underexposed, so that you don't create too much streaking (if that isn't your intention). I shoot with the flash in manual mode and +/- depending on my desired outcome. Example: