Wow, great looking shots and you're getting some fantastic blur in the background. Sweet!
As for the tilt, I've found that the angle of tilt depends on what part of the car your shooting. For instance, if the car is coming head-on you can make the tilt a bit more extreme but for side views just a slight tilt seems like enough.
But it looks to me like you took the time to experiment with different tilt angles so whatever looks good to your eye, go with.
The best way to see if you've got too much tilt is if you find yourself tilting your head to look at the shot. I didn't really tilt my head for the first one, but I did for the second. But the shots themselves are fantastic, especially with the motion blur.