I never use the flash for shots as I don't like the fallout you get with the light with the subject being nicely exposed and the background black/underexposed, also I'm not a fan of racing with people using the flash.
I find basically to take indoor shots it will take a lot of practice to start getting a high keeper rate and you cant expect to get any noise free images unless you are using one of the very latest batch of camera's and even then I wouldn't expect miracles.
I think accepting what limitations there is and knowing that it's very unlikely you are going to get something that will stand up for more than web use or a small print. There isn't really much point expecting to get clear noise free images that are perfectly sharp at 100%, It's just not gonna happen with most indoor lights.
Focusing I generally pre-focus then take the shot when the car is in the plane of focus as the AF indoors generally isn't quick enough to keep up with the cars if they are coming towards you or going away. Panning shots you can often use the AF if they aren't changing much distance wise. With my 20D and 70-200 f4L I find I can AF as long as they aren't within about 3-4m of me, any closer than that and the lens cant physically move the glass quick enough to keep them in focus so if you're looking frame filling shots then it's manual focus.
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