This looks like one of those shoots where a huge big reflector board would have been useful to bounce light up into the shadows. That black car just makes it difficult to balance the exposure with white clouds.
In the second picture, why did you plant two trees and a sign in the engine compartment? :er:
Portraits of cars, and people, should not have things growing out of the top of them, just too distracting IMHO. I would also agree that the sun was not your friend and you could have really benefited from a large reflector or the sun coming more from behind and lower.
In the case of sun behind and lower, meaning like sunset time?
I will keep in mind next time to double check the stance of the shot as any object behind the subject can possibly be a distraction point. If I were to offset the car against the sign and trees would that be an ok shot?
I guess I was enjoying the car (since I have a black vette also) and not paying close enough attention to the other details. Yes, I agree with what others have been saying in order to improve the shots.