With an Ecoboost engine, that is not exactly true. Peak efficiency & HP/TQ numbers are published by Ford by using 91 octane gas. Using gas with lower octane will not only reduce peak HP/TQ, it will also adversely effect FE, yes fractionally, but it's still something greater than zero. The percentage of ethanol in the gas will also affect FE, independent of octane differences.Octane will have zero effect on your fuel mileage. Take it from a petroleum engineer.
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