Done entirely in Photoshop. I'll generally start with a loose sketch like this. When i use a more painterly technique, the sketch is mostly a loose guide, and will eventually be turned off. It can be very helpful to flip the sketch horizontally as you work; it will make perspective errors much more visible.
Making a new layer underneath the sketch, start to block in the basic colors and values of the vehicle. Don't worry about precision at this point, it is more crucial to get a good read of what the surfaces are doing. Here I used a hard brush with "other dynamics" set to pen pressure (you can play around with this in the photoshop brush menu), but I'll occasionally use a soft round airbrush for this stage.
Next I'll start to lay down some value on a new layer on top of the sketch to start covering up the lines.
When the basic structure is completed, start to define the details like wheels, basic highlights, etc.
Continuing to refine details and highlights- decided to put a roof on it, so i just painted it on with the hard brush. For the sharpest highlights I've used the lasso tool to create a hard edge, and with softer highlights I've just painted it on. Its important to think about what kind of highlights the car will be showing, and how the environment reflects onto each surface of the car. It makes it more believable at a quick read, and looks cooler in my opinion.
At about this stage I'll turn of the sketch and see how the values and colors stand up with out it.
Put on the finishing touches: make details crisper, correct errors in value, etc. Finalizing highlights here- reflections at the edges of the vehicle tend to give it more wrap and place it more realistically within the environment. Finished!