Doesn't sound like an octane problem but, an Ethenol problem. Octane won't affect your mileage much however, Ethenol will. Ethenol has a much lower energy rating, meaning it takes more to make the same power.
2011 Le Mans Blue M3// Black extended Novillo w/ Blue Grey Aluminum trim/DCT/ZTP/ZCW/ZPP/ZCP Premium Sound ///Akrapovic Evolution Exhaust
2012 Le Mans Blue 135i M Pkg DCT
2013 Ford F150 FX4 Supercrew