I assume that the mileage checks that you do on "pure" gasoline are high octane gasoline.. see Insider's comments above..

short answer.. we don't have enough "pure" gasoline to go around. 

Your 400 m b/d stocks figure is "only" about a 21 day supply.. assuming it's not all Heavy Junk which we don't have the capacity to refine anyway.. again see Insider's comments above..

Here is the link to the article that said that in 2011..

"ethanol reduced wholesale gasoline prices by an average of $1.09 per gallon."

http://www.ethanolrfa.org/news/entry/new-university-study-ethanol-reduced-gas-prices-by-more-than-1-in-2011/

A couple of weeks ago "we" were cranking out 900 k b/d of etoh.. If by your own admission that etoh offers 3% less mileage at the 10% blend rate.. then we "only" need to find 630 k b/d of gasoline to replace it.. the Bakken (North Dakota) is currently cranking out about 600 k b/d.. of which about 44% can be turned into gasoline..

Lol..

"So" we "only" need to find 2.39 "more" North Dakota's.. uhum.. to replace ethanol... 

and it only took us about a decade to get North Dakota to the 600 k b/d level.. (side note: we also need a pipeline to get it FROM North Dakota to the refineries..)

How high do "you" think gasoline is gonna go...????