Did some research.

Hemp and Gasoline

18.1 % of the landmass of the USA is arable land. That means we have 671,510 acres of farmland.

In 2010 we consumed 137,970,000,000 gallons of gasoline.

An acre of land, planted in hemp has an estimated yield of 500 gallon; there are 640 acres to a square mile, so a square mile of hemp would yield 320,000 of gasoline equivalent.

It would 431,156,000 acres of land to produce enough fuel to match our consumption in 2010. This would require 64.2% of our arable land

I think the collective memory of the general public is at the outside 6 months. This makes rewriting and disseminating this freshly minted (defecated) bull (shit) to a wide and accepting audience simple.