These were taken at a WERA race here a couple of years ago.
This guy was probably doing 60-ish after coming off a corner and starting to accelerate down a short straight on a road course. I shot it at 1/200 because I was trying to catch a blur of the bike in the background. It was shot at 125mm and not cropped much, I was probably within 30' of the bike as it came by me.
This guy was probably, at a guess, doing 150 or so. The shot was taken at the end of the long front straight going into the first curve (high-banked NASCAR-style curve) and he was hard on the brakes. You might have noticed that the rear wheel is off the ground

It was shot at 180mm and 1/320 second and not cropped much, so I was probably within 50' when the shutter opened.
The point of both is that shutter speed is dependent on distance, but even a relatively fast shutter speed will convey motion. That close, even with a relatively fast shutter speed, you can get really good motion blur yet keep sharpness on the subject. The first time I went to that race I shot a lot faster and was disappointed with the results. This one was taken at the exact same spot as the first one but rather than 1/200 second it was shot at 1/800. The background is frozen, the spokes on the wheels are frozen, and it just doesn't convey the motion as well. Play with your settings next time and find what works best for you, but I think you'll get a little better sharpness on your subjects with a bit more shutter speed.