No, the reduced energy content means less MPG. You should get the same or MORE power on E85 since it has higher octane (like >100) and ethanol drops intake temps more than gas. Even if the FFV doesn't turn the timing way up when the ethanol content is high, it will at least get rid of the knock retard that most stock vehicles see under certain conditions.
All FFV have a fuel sensor to determine the current ratio of gas/alcohol and scales the injector pulsewidths to get the correct AFR. Ethanol needs about 9:1 while gasoline needs 14.7:1, so E85 is like 9.7. That's your reduced MPG right there. Same thing applies for E10 or E15 in a regular except that they don't have a fuel sensor, so the mixture is wrong until the ECM has time to trim the values to compensate.
All FFV have a fuel sensor to determine the current ratio of gas/alcohol and scales the injector pulsewidths to get the correct AFR. Ethanol needs about 9:1 while gasoline needs 14.7:1, so E85 is like 9.7. That's your reduced MPG right there. Same thing applies for E10 or E15 in a regular except that they don't have a fuel sensor, so the mixture is wrong until the ECM has time to trim the values to compensate.

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