Crude Oil Board

Sorry if this post is off topic for this board, but I have a question about refining that I simply cannot find the answer to anywhere.

I understand the term "fractioning" is the distillation of the contents of (a barrel of) oil into various products including gasoline, diesel, etc.

What I am I are trying to understand ... is ... can one distill an entire "barrel" and turn 100% (assuming 100% efficiency) of its contents into, say gasoline.

Or, is the barrel divided into layers? Where only, say, 38% can be used for gasoline?

Reason I ask is ... if 1,000,000 vehicles convert to alternate energy supplies, do we end up with a glut of unused (pre fractionated / crude?) gasoline or does the industry refine less crude?

Thanks

Robert