1)
A car is probably one of the most difficult object to scan, because of reflective surfaces (windscreen, glasses, etc.) and poorly textured surfaces (most of the metal body). You will get better results on a (very) dirty car, covered by mud. Alternatively, you can cover it with powder, confetti, or anything to generate some reference points (and to mask reflections).
Usually, tyres are very well rendered.
2)
20 images around an object is probably not enough. I would recommend about 50 around 360°, and if possible, more than one circle around (say one at ground level, one at 1m heigth and one at 2m).