http://feerc.ornl.gov/pdfs/pub_int_blends_rpt1_updated.pdf http://feerc.ornl.gov/pdfs/pub_int_blends_rpt1_updated.pdf Here are some fairly extensive test cycle

http://feerc.ornl.gov/pdfs/pub_int_blends_rpt1_updated.pdf http://feerc.ornl.gov/pdfs/pub_int_blends_rpt1_updated.pdf Here are some fairly extensive test cycles applied to real-world engines, which found that the effect closely mirrors the theoretical reduction in energy density (3-4% for E10, ~7% for E20). Higher octane ratings are not generally beneficial for fuel efficiency unless the engine has been designed for them (compression ratio, cam profile, timing, tune). Dropping higher-octane fuel into an engine designed for a lower-octane fuel will generally not affect economy.