ISO 800 may make the image a bit too grainy. Definitely take a few sample shots, beforehand, with different ISO settings, then adjust the shutter speed, once you're satisfied with the ISO. As Stereo suggest, something around 1/1000, which should be really easy to accomplish in direct sunlight. Also, if you can, be sure to have a very wide aperture (f/5.0, or so). Exposes the sensor more and will let you use an even faster shutter speed. It will also give you a nice, shallow depth-of-field that will make the cars stand out from the background, specifically while zoomed in.
Too bad your sport setting doesn't seem to work. The sport setting on my XSi is fantastic. Live auto-focus for constantly moving subjects, shutter speed optimization for the fastest shutter speed possible, up to 3.5 shots per second, and it even makes colours a bit more vibrant.
By the way, that shot is horribly grainy.
![Sad :(](./images/smilies/icon_sad.gif)
Don't suppose you have specs available - ISO, shutter speed, whatnot?