I would shoot manual, for two reasons. Firstly, you need to control the shutter speed, and secondly, because you will probably be panning and tracking different cars a lot of the time, you need to lock the exposure.
If you use any of the auto modes, when you're shooting a white car and then a red car and then a black one, then your exposure will vary when it shouldn't. Likewise, when you're panning, the background will change and as it moves from grass to shade to perhaps a bit of sky, then your exposure will change as well. And unless the light actually falling on the subject has changed, the exposure should obviously remained fixed.
Only use an auto mode when the sunlight is changing more than the subject, ie when it's cloudy and windy.
For metering, the pro's choice is usually spot. Take a reading off the grass, or some other large area of medium tone that you can go back to and reference any time. Grass is usually favourite because it's close to mid-grey and there's plenty of it about. Set the exposure, chimp it and check and modify as necessary, note the variance of the viewfinder needle from zero, ie say +0.3. This is now your reference.
If the light changes, you can go back to that reference tone at any time and so long as you re-adjust everything to reset it at +0.3, then you'll always have correct and consistently identical exposure. When you see professionals pointing their long lenses at the grass or tarmac for no apparent reason, that's what they're doing.
