Then you are doing it right.
YOU HAVE TO PAN.
So keep doing what your doing.
Make sure to keep the motion for a second after you hear the photo take. A lot of people hear the click and lower the camera, and it creates a touch of blur, so follow through on the motion.
You want to be the same distance away from the car for the whole pan, otherwise as they corner gets a little closer, they may end up out of focus. If the corner is a little closer on part of your pan, make sure to rebump the focus on the camera while firing off pics.
or focus manually if you think you can
the closer to cars, the better usually.
I shoot my setting in full manual. and i just over expose a little. Its digital, if its close to blown out, it looks like shit. So I just let it go and make sure the focus point, (the car), is exposed well.
And a brighter photo will pop out more and have better energy then a dim one.
If you are having a hard time getting the cars in perfect focus, turn the shutter speed up just a touch.
This is just my opinion, as I dont consider myself a photographer, i just like to play around.