I was shooting around 1/125th to make sure the car was crystal clear. I was shooting these for prints. That, and most of the cars were not stopping for photos so that made it more of a challenge. I missed a lot of shots.
I was doing double exposures using bracketing. One shot at 1/125th to freeze the car, and one shot at a slower speed to blur the wheels and background.
Unforunately, the bracketing was working when it wanted to...new camera time!
The faster you go, the better results you will have. Just try to keep both cars steady, and don't drop the shutter speed TOO low.
Thanks for the heads up. Brought myself a Canon 40D last week and that should sort ort som practicals I've suffered. Do you know if its possible to take dual exposures with one click. Like the way you mention the theories it would be helpfull indeed.