Car (and in most cases, everything else) photography 101:
Photography is about light.
Vertical light is bad - it is harsh, casts shadows, and washes out colors with intense contrast.
Horizontal light is good - find this 1-2 hours before sunset.
Take photos with the light source behind you.
For cars, take photos at waist height, and experiment above and below. "Top down" shots rarely do a vehicle justice.
Quartering shots often look best. When taking shots of either side of the car, turn the wheels
away from the camera to expose the rim, not the treads, to the camera (as was said).
Good luck...