For what it's worth, I think you will find that the average acre of corn in the US will now produce about 437 gallons of ethanol. The 138 bushel per acre yield and the 2.7 gallon per bushel distillation rate are based upon 1996 - 2005 averages. Both the yield and the efficiency of distillation are increasing at a pretty good clip.
This past year seems to be at 151 bushels per acre and 2.8 gallons per bushel for a total of 423.
The corn yield is from the link above for US 2007 harvest and the 2.8 is in the most recent RFA’s Ethanol Industry Outlook 2007.
http://www.ethanolrfa.org/objects/pdf/outlook/RFA_Outlook_2007.pdf