The other main consideration is the ISO. If the light is low you will need to increase the ISO, effectively increasing the sensitivity of the camera sensor. This enables you to gather more light allowing you to close the aperture down to a smaller hole, which will increase the depth of field (DoF).
This increase in DoF will make more of your picture in focus. A large f/stop like f2.8 focusing on a subject at 25m using a focal length lens of 200mm will only have a DoF of 1.74m. A smaller f-stop like F8 will have over 5m of the subject in focus. This can be the difference between getting the front and the back of the car in focus or not.
Having said all that, having the aperture as wide open as possible will give you a nice out of focus background. Only you can determine what settings will give you "that" picture that can make the trip worthwhile.
It goes without saying that the auto focusing (AF) mode is also important. There will be a number of modes on your camera. The best for motorsport will be continuous AF. Combine this with back focusing and you will see the camera track the subject, continually changing the focus, so when you press the shutter the image is sharp.