For those that insist that higher octane provides higher fuel mileage - ponder this...
The EPA has mandated that manufacturers reach a Corporate Average Fuel Economy average (CAFE for short).
Here is a graph of the mandated average fuel economy required by year.
One thing about the chart is - you need to understand the meaning of "vehicle footprint".
If simply changing to higher octane fuel would boost fuel mileage as some here want to claim - the factory would specify a higher octane rating.
They don't, because higher octane doesn't provide fuel mileage gains.
For those of you with a modified tune in your car - you very well may need the higher octane for performance reasons.
CAFE has nothing to do with efficiency and power output of an engine. CAFE is a legislated method of the US government to require a manufacture to meet a fleet MPG across their entire production output in a given year. For example, if GM wants to place a specific allotment of Silverados on the road, they must offset that production with a higher MPG vehicle, like the Sonic, Cruze, Volt, etc. GM must meet this average CAFE for each production year as time and restriction of the target EPA/DOT MPGs progress.
Higher octane does not boost fuel mileage directly. It does allow timing to be advanced, higher design compression ratios, and boost pressures(turbo) to be increased. This is achieved by the increase in the resistance of fuel to burn as octane increases. Without getting much more in depth in combustion cycles of a internal combustion engine, ultimately... the further the ECM controls push the combustion cycle based on the oxygen content of the air and the timing of the valves opening and closing, the more power you will make and the more efficiently you will burn the fuel. With this in mind for a hyper-miling scenario, the more power that a car can put down without increasing overall consumption of fuel when load is placed on the engine, the less fuel will be used. Another method of increasing output while decreasing consumption is direct injection. It allows for fuel stream to be directly injected into the cylinder at a much lower temperature and potentially more effective atomization pattern for complete combustion. With this in mind, a car with higher cylinder pressures will run with lower octane with little-to-no knock for the same power output and efficiency. (This understanding is consistent with methanol and water injection.). E-85 has a similar effect but it's ability to burn much slower than even a high octane gasoline. This is generated from the lack of stored energy(volatility), also referred to energy density. The lack of energy will be reflected in the use of fuel. Gasoline has a stoichiometric ratio of 14.7:1 and E-85 is 9.76:1. At the same RPM and load, an engine will use ~63% more fuel on E-85. Tuned engines that push design power output parameters (that are defined by the producer to provide long term operation without failure) can be pushed to greater output levels with DI and E-85, due to the ability to further perfect the efficient combustion of the fuel source.
In the end, the driver's foot and the brain connected to it ...is the most powerful force available in altering a MPG outcome. If you can't keep your foot out the pedal, you will use more fuel than the next guy/gal. If you want better MPGs, please don't drive over 2500 RPMs and use any gas you want. If you want to utilize the engine for its true capabilities, please consider how your driving style fits the load that you place on the engine. Does that load create a scenario for consistent knock and wear/tear on the internals?
Food for thought on the wear/tear point, why would GM put a TSB out on the 1.4T ...requiring(when consulting with a GM service desk) in summer temperatures to run high octane, especially in areas of 90+F where the temp and A/C are tasking the engine?
The 1.4T engines ability to adjust to knock on 87 octane cannot be overcome in those conditions.
Have a great weekend!