Ethanol has close to 34% less energy per volume than gasoline, consequently fuel economy ratings with ethanol blends are significantly lower than with pure gasoline, but this lower energy content does not translate directly into a 34% reduction in mileage, because there are many other variables that affect the performance of a particular fuel in a particular engine, and also because ethanol has a higher octane rating which is beneficial to high compression ratio engines.
Ethanol has about 34% less energy per volume than gasoline, which results in lower fuel efficiency ratings for blends compared to pure gasoline. However, this does not mean a 34% reduction in mileage; other factors and the higher octane rating of ethanol can improve performance in engines with high compression ratios.