In addition to above posters, lower octane fuels usually contain alcohol to cut cost(in southern Ontario its usually 10% for 87, 5% for 89 and 0 for 91) , alcohol has less energy than the rest of the fuel and it may also burn more (look at fuel efficiencies of cars running on E85, they're terrible, for example dodge avenger is listed as 10.8 & 7.2 l/100km for city and hwy, respectively while using regular unleaded, and 15.1 & 9.7 l/100km on flex fuel, that's even more than my m35x!) however i am no engineer so my logic may be flawed.