= = = You had official numbers for the same years, and then added an unofficial number 9 years later. You then compared people per household in 2001 to vehicles per household in 2009. Why? = = =
Because the first set of numbers came from US DOT which has not published an official number for 2010 yet; the 2010 value is from a widely-quoted private source of statistics on automobiles but is not an academic quality publication.