If it is the quickest shutter speed that you camera has... then there are 2 things that may happen...
The first is that your camera is not fast enough to take such pictures. What model is it?
The second is that you mat not have enough light to take a picture in the maximum shutter speed, and your camera automatically compensates by reducing the shutter speed.
But you can take a nice pic of moving objects without blurring the object (the background will stay blurred). It's like this... you follow the object for a few seconds before pressing the shutter, and this way your lens will have the same angular speed has the car, and the car won't (If well done..) blur.
Let me know if it worked!