I LOVE race car photography...The color, pit activity, smell of racing fuel, hot brake shoes, the scream of a Porsche engine as it passes at about 150 mph, and the growl of a Corvette at about 147; even the pit guy that hollers "watch it, photo guy, car coming!"
What a day it was! Beautiful,clear weather! An overcast to lower the contrast would have been nice, though. I did mangage to catch wheel lift on a most unlikely (at least, I thought) car. The little black one, with a star on its side.

The Iron Duke, my ooold Nikon 70-210 performed like a champ!
Some miscellaneous ramblings re: exposure, shutter speed, camera support and sharpness.
My hand-held stuff at 1/200 and 1/250 was sharper than my monopod stuff at like shutter speeds.

Hmmmmm..... Learning curve? Probably. My little $7 Chicom ball head has been summarily demoted to holding my flash units on the light stand. I may take another look at the $maller Manfrotto 'head.
We can talk about glass, focus, and other factors all we want, but, even with the sharpest glass, suficient shutter speed, exact focus, proper stance and follow through... BUT, and this is a biggie, we cannot control what the car is doing! Not in a million years, ever, as much as we might wish it were otherwise. I did manage to slightly blur a car at 1/1000 sec yesterday. I know: ol' fuzzy-focus is at it again!
I dislike the cars looking as if they were butterflies pinned to a board... tack sharp from stem to stern, static. Unless they are sitting still, which they seldom do during a race. The one exception was a Ferrarri that got frightened off the track by all those rowdy, noisy, cute little Miatas yesterday.
Despite all, except in a studio, tack sharp is largely a matter of luck!
Sports mode gave mostly very sharp pixes, and the autofocus worked marvelously, as it should, but the cars were experiencing engine vibration, suspension jiggle, driver input, road bumpiness ...dancing around, Hence, my slightly fuzzy, but not unacceptable, sharpness at 1/1000. (That's my story, and I'm sticking to it!)
Depending upon the speed of the cars, even when they were really racing, panning with 1/500, and 1/600sec. showed just a smidge of wheel movement. 1/400 was close, and 1/320 - 1/250 were good, providing some unsharpness in the background, too.
I had a really hard time getting wheel movement on the Ferrarri, even at 1/160 sec.

He shoulda been driving a bus! I mean, being passed by Subarus? It just ain't right!
Anyhoo, the short version is, I seem to have found out that panning with any shutter speed from 1/80 sec.can provide the desired wheel motion and background separation, given proper attention to detail, but the pixes get sharper the higher I go, and 1/200 is mostly good. For air shows, 1/250 will provide some motion in propellers. You remember, the fan thingies on the pointy ends of ze planes?
But! when the wibbles cancel the wobbles, or we don't sneeze or jab the shutter release, or caffeine hasn't taken its toll, or the moon is in the seventh house, or something, sometimes our pixes will be very sharp, but if they aren't, sometimes we are not totally to blame, ( NOT my fault, man!) but most our photos will be acceptably sharp, and others excellent.
Now get out there and take some pixes!