The initial thumbnail sketch was about 2 minutes, and 5cm small. With 1 hour of work, I tried to take it to a level of possible evaluation by other people.
Here the sketch was scanned, and the wheels were made perfectly circular. If something that the viewer thinks should be round is perfect, it is easier to believe the rest of the shapes to be perfect too, although the viewer has no clue how the other lines should go.
The base color of the car was chosen to be green, but adding reflections and glare from the sun brings the car body alive, and makes the sketch more dynamic.
Making the background work with the car brings it into the environment. This is importnat: cars are always seen in an environment.
Spending some time on examplary details gives the idea of the image being detailed in every place. A great trick to use when the time is limited. Notice how the door handles aren't given much attention at all, whereas the wheels are taken to the detail level of tire treads showing. The white highlights also create the high contrast dynamism against the black background. High contrast = good.
Here I added a person. In car sketches people are not that important, and I usually don't draw them, but characters seen with the design give it some scale. Especially useful when designing alien spacecrafts, big boats, buildings, or other objects with unknown size.
For the final step, I increased contrast and played with the color loevels, and also added a gradient to darken the back side of the car. I did this to lead the viewer's eye to the front part, which I want to pay more attention to.
The beauty of working in this way is that I could go on forever, or at any given time call it done and give it to the client. It's a matter of time. In this case, I decided to stop after 1 hour and work on other designs.