It's pretty obvious; everybody says it's China. But here's the graph to show it:

In case you want that deciphered easily, oil consumption growth is declining in OECD countries, and growing (massively) in Asia / China and developing countries.

(Here's the OECD country map.)

The cynic might say that no matter if we in the US all buy more fuel-efficient cars - plug-in hybrids even - and drive less,it won't make much of a difference.... That wouldn't stop me from buying one. But still.