That addresses the numerator, but what about the denominator? How is "household" defined? If "one household = one koseki (family registry)" then the number may be very misleading. It would not surprise me if koseki was used to define household, as this would be an exact number that was readily available to whomever compiled the car ownership statistics. But isn't it true that most young adults remain on their parents' koseki until such time that they get married? If so, then most single adults, many of whom live by themselves, would be counted as part of their parents' household and not as their own household. While comparing the changes in the ratio between different time periods would be equally useful no matter how it's calculated as long as it's consistent, trying to make sense of the ratio in isolation (such as by observing that the number seems high) may be in conflict to what you'd think it might be, depending on how they define household.