Hi mkilgour

There are two different subjects here: exposure and motion blur.

Exposure. Because of the contrast between the body and the wheels, the easier way is to expose mostly to the body and process the wheels exposure. There are many ways to do so if you work in raw, the brush tool in Camera RAW it's quite simple.

Motion blur. If you want an action shot like this, the background, the track an especially the wheels must be moving. The more motion blur, the more effect of speed. For panning, select Tv in your camera, start with a speed a little less than the focal you are using: if you're going to use a 200 mm, select 1/160 s. Once you get a valid take, change to a slower speed (1/125 or even less). For these shot, a tele works much better than a standard lens.

If you want to show the detail of the wheels, you'd better take a still picture. It makes little sense taking a shot of a moving subject and freeze the movement, unless you want to show something you can't capture if the subject it's steady.

In every case, try to give more space in the front of the car than in the back. Like in a portrait, giving space to the "sight" of the car usually produce better results.

In panning, you can start in continuous drive. Then try single shooting, you'll have only one picture every time the car pass in front of you, but you can frame exactly the picture you want (composing, background) if you do so.

Please excuse my English. I know it seem like I'm commanding, but it's only lack of lexicon.