http://www.biodiesel.org/pdf_files/Biodiesel_Blends_Above%20_20_Final.pdf
I'd be willing to bet if energy/gallon were critical, they're be a few PhD ChE's who'd develop ways to close that gap. But I'd guess the real issue is energy/mass since how much mass you have to haul around hits your econs more than stretching a fuel tank's size a bit.