Japanese cars were better because we bombed their infrastructure so badly that when the war was over they had no option but to rebuild with new equipment, while our auto manufacturers were still using prewar equipment. They had a jump on us, because our products were too effective.
Acutually if you ask any Japanese what made their quality and manufacturing superior it was because of a single American Man: W. Edwards Deming http://en.wikipedia.org/wiki/W._Edwards_Deming