Quote from waxwing:
What would happen if the US cut consumption by 15%?
Well, with all other things being equal, and given that the 5% of the world population in the US consumes approximately 25% of the world's production, then the obvious answer is that consumption would be reduced by 0.15*0.25, i.e. 3.75%.
Unfortunately, all rational analysis seems to suggest that production cannot increase, and is likely to fall from here, and that demand from Asia is rising rapidly.
Therefore I'd say a reasonable answer to your question would be that if the US cuts its demand by 15% very rapidly (months, not years), it will delay a further rise in the price of oil. For a little while.