I have some similar racing work
here, and earlier in that thread.
My pics seem a little "flatter" than yours, and I'm not sure if it's not because of my monitor at home, or if it's because the D50's sensor sucks. They look great at home, then I get to work and bring them up and they look a bit dull. I'm still working on the smooth panning, but to get the motion I want I use a fairly long shutter, 1/125, even 1/60, which at 200mm is an eternity. I don't have the best lenses, either, just the kit lenses the camera came with (18-55 and 55-200.) Saving for the 70-300 ED VR lens from Nikon, which is at the top end of what I can spend on glass, and I've replaced the D50 with a D5000, but haven't been to anything with it, yet.
Regardless, I enjoyed perusing your pics. That Muscle Milk Aston sounds incredible, doesn't it? And the camera can't do justice to the green of the Tequila Patron Ferraris, either. That green hurts in real life!
Your pics have much more saturation, to my eye maybe
too much, and they have a lower luminosity. I'll play with that with my pics, as soon as I get some to play with. (Thinking about Petit LeMans, and in Novemeber will take in the Blue Angels homecoming air show in nearby Pensacola.)