I recommend this book. I have it and it's phenomenal.
How to Digitally Photograph Cars (Performance How-to): Jason Siu, Josh Mackey: 9781932494990: Amazon.com: Books
I usually use manual mode, 1/30th - 1/100th of a second shutter speed, adjust your aperture as necessary to the light situations (sunny, overcast, dawn, dusk, etc). The slower shutter speed, the better the background and wheel blur. But too slow and you will get a blurry photo from camera shake/movement. 1/40th or 1/60th makes a really nice photo.
Also, set your camera to AI Servo, burst mode, and if you're leading/panning, set your focus points to lead towards the front of the car, centered doesn't always work...you can shift your focus points from center to left, right, top or bottom. play around with it till you get the best results.
This was 1/60th, 100 ISO, f/9. shot on an early summer morning late August of 2012 with a Canon 7D and 70-200mm f/2.8 USM II IS lens.
Canons sometimes have trouble focusing on the outer perimeter; for pans, I always leave it on center unless I'm really close, shooting car to car or shooting at a higher speed (1/80-120). Just reduces the risk of the camera failing to focus in time or ending up with a "soft" shot. Most of my rolling shots are standing pans, so center works best.
The hardest thing about pans/motion shots is maintaining decent composition with a moving object. Ideal IMO is to leave enough background to frame the subject, with more room "in front" than behind to conceptualize freedom of motion. Using a rule of thirds, object placement on one of the lower corners works best for me.
5D MkII, 70-200 2.8L, 70mm, 1/30, F/14. Standing pan.