Here are some of my panning shots at Laguna Seca - Monterey Historics:
Aston DBR1
Ferrari Testarossa
Maserati Birdcage
In general, I find that I can get the panning shot best if I tilt the camera about 45 degrees to the left or right so thet the car is in the middle of the diamond and follow the car for quite a bit so that the camera can get a good read of the car's speed. The first two pictures are using this technique.
I use the automatic settings and pray for the best.
NO manual settings - I am a neophyte
I find this technique really helps with filming F1 cars that are so friggin fast, half of my shots have only parts of the car and not the whole because I was too slow to keep up with it.