TBH, when I'm doing panning shots of vehicles in motion, I rarely use AF. Normally I prefocus on an area on the track that the car will pass though and use this.
As Andrew said in the second post, panning is the technique. you need to master. Looks easy but takes some practice. The mistake that many people make is to not smoothly following the action BEFORE and AFTER releasing the shutter. Learn to follow the action and smoothly pan and press the shutter (motor drive is good for this) then keep on panning after you have taken the shot. Also, squeeze the shutter release, don't stab at it.
Start with shutter speeds of about 1/250 to 1/125 sec to start with, until you have mastered the technique, then when you are confident, start lowering the shutter speed. Depending on the focal length of the lens in use, aim to drop down to somewhere between 1/60 to 1/30 second for that ultimate creamy blurred background. Don't forget also, if you are shooting with a stabilised lens (VR for Nikon and IS for Canon), if the lens offers it, set the VR mode to "Active" from "Normal". In this mode the camera assumes you are panning or shooting from a moving vehicle and therefore only looks to reduce shake in the vertical plane and not the horizontal (panning) plane.
Choice of lens focal length is equally important. Depending on how far away from the action you are and what type of camera you are using (I.e. a DSLR with a cropped or full frame sensor, or a bridge all in one camera), I normally use either a 200 or 300mm lens and use my cropped Sensor DX camera to give an effect Focal length of between 300-450mm. Remember, the longer the focal length, the shallower the depth of field will be (the area behind and in front of the actual plane of focus that will be rendered sharp), so long lenses are good for defocussed and busy backgrounds. However long lenses tend to be more difficult to handle and are generally heavier. Therefore my final piece of advice would be to use a monopod (not a tripod). The monopod will allow you to pan even more smoothly and also take the weight of the camera/lens combination making it a more comfortable experience.
As with most things, practise makes perfect.
