Well, I have a little different opinion.

1. In general higher octane will not affect the efficiency of your engine or give you better mileage, but...
2. If you have a newer car that calls for high octane gasoline, and you put in low octane instead, the following will happen. The computer will detect knocking with its knock sensor and retard the timing. Retarding the timing will reduce your max availible hp/torque and will reduce the efficiency of your engine. Thus, your mpg may go down if you use lower octane gas.
3. Some econobox cars have engines with knock sensors. While most economy cars specify that only low octane gas is necessary, the cars computer may adjust if you put high octane in. It will advance the timing, thus giving you more hp/torque and efficiency under some circumstances. I believe the GM ECOTEC engine used in Saturns and Cobalts, etc has a knock sensor in all versions.

Typical blog huh? Everyone has a different opinion.

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