I look at the octane as a stablizer. The higher, the less likely to have detonation. Summer blend gasolines will have between 113,000 to 117 or 118,000 BTUs. Winter blends 109 to 114,000 BTUs. You'll only get so much energy from gasoline. The higher the compression, the more octane you need. Mileage is all in the tuning, the riding style and areodymamics of the bike.