Do you really need to shoot in a moving vehicle, or do you just need to present the effect that the car is actually moving? Many people opt to do the green screen thing, it's cheaper, and safer, and you can control all of the lighting, sound, etc. Video Copilot.net has an awesome tutorial on how to do this, it works! I just did a scene last weekend where I have my kids driving my pick-up in a high speed chase, just to experiment with the tutorial and mess around more with my green screen. My kids loved it, but want to do the chase scene for real, not gonna happen!