this is what I would go to give it the "speed" effect. I use CS3 too

1. duplicate your render of the car
2. with the top layer selected, go to Filter > Blur > Motion Blur and use these settings: Angle- 0 Distance- whatever you want, depending on how fast you want the car to be going. don't go too high though, since it might look unrealistic. try staying under 100. Hit OK
3. Change the opacity to under 100%. The slower the car is going, the more you should lower the capacity. The faster the car is going, the less yuo shuold change the capacity. So if your Distance was 50 pixels, I'd recommend making the opacity around 70-80%
Hope I help you
