Originally Posted by cudase
By law, if the pump says the fuel is a certain octane, it has to be that octane or better. Also, adding ethanol increases the octane rating of fuel. The reason you get worse mileage with fuels containing ethanol is because ethanol has a lower stoichiometric ratio than "gas", and the o2 sensor detects this and will increase your fuel consumption to get your ratio closer to ideal.
Ethanol also has a lower energy yield so you will find yourself using more throttle.
maybe this is true where you live, but in jersey its +/- 2 for octane levels.