If you want to photograph a car and make it look good, then I say you should wait until dusk. There's something about the colours and reflection of the sky on paintwork that shows off the shapes and curves particularly well.
Here's an example from last year. We took the big Merc SL500 to a corn field on the top of a hill one evening. There were no buildings or trees nearby so wall to wall sky shows off the shape quite well.
I suspect a clear sky would have worked more effectively, however the clouds add a certain drama.
Here is another (rather humble) subject photographed against a cloud free sky at dusk. Here the gentle graduation of sky colours work their wonders on the painted steel. In this case I was lazy and left the car near the local trees, which add and subtract from the image in my opinion.
Reflections of trees and buildings do give important visual cues in these circumstances, but they can also be distracting.