Car Sales, Gasoline and Demographics

It's no secret (at least it shouldn't be) that gasoline sales have plunged. Here is a chart from my April 6 post Another Plunge in 3-Month Rolling Average of Petroleum and Gasoline Usage for Jan, Feb, March 2012
Jan-Feb-March 2012 petroleum and gasoline usage vs. the same three months in prior years.

click on chart for sharper image

Every month for quite some time, the rolling average of petroleum and gasoline usage has been trending down. The question is "Why?" Some pin this on car mileage improvements but that answer is easy to discredit. Fuel efficiency has been rising for more than a decade, but the plunge did not start until the Great Recession in 2007.
However, the Great Recession is over, yet gasoline sales have not rebounded. Is this an indication another recession is on the horizon? That the recession never ended? Something else?  (more)