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Oh, were life that simple!
There's more to this question than you've given us data on, so we have to make a few assumptions.
Let's assume that you don't want to get run over, so you're using a long lens, say 200mm on a DX format. The front average car will fill the frame at about 75ft at that focal length. Again, assuming that your aperture is around f11 the depth of field will be from 66ft to 86ft if you focus at 75ft. This means the image will be acceptably sharp as it moves through that 20ft DoF. So, in theory you could shoot at about ¼ sec and the object would be in focus. However, of course, an object moving at speed will be subject to speed blur at that sort of shutter speed and that will be a far greater problem than focus.
If you're shooting hand-held with a 200mm you should be shooting at about 1/200 sec unless you're using VR, in which case you could realistically get away with 1/50 sec if your stance and camera holding technique is good. Practically, I would tackle such a task at about 1/250 sec, then I would switch the AF off and pre-focus on the 75ft mark and wait for the car to enter my zone of focus, possibly shooting a burst.
Hope this helps.
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