A handy hint is to make sure you only have the center focus point selected, then the cameras focusing is just working on that one point rather than trying to calculate for 5 or 8 points etc.
I have always used Tv when i did the motorsport stuff, and if i cannot get the shutter speed i need i just up the Iso, these days noise at the higher iso settings is less of a problem.
Also try picking a slow corner on the track as it will be easier to track the car and get it in focus.
The attached was shot from memory at i think ISO 800 just before a big downpour, with little noise in the photo.
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Jonathan | Atomic-Carbon
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