I'm working from this original image, part of a bracketed series, and plan to create a composite with a new background.

Here, I've lifted the entire car from the background. I used the pen tool, which is great for objects like this with clearly defined edges. I'll pick up the shadows and ground later as needed.

FIrst cut at adding the new background. Sky merged with the original ground, and some reflections added in the windshield and the surface of the car body. Also, a bit of color correction to match the three elements together (ground, sky, car).