Interesting to note, MTBE has about 80% the energy of gas, while ethanol has about 67%. So moving from an MTBE mix to ethanol is about the same drop as moving from straight gas to MTBE.
http://www.epa.gov/otaq/regs/fuels/ostp-3.pdf
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As can be seen in Table 3-1, the theoretically expected decrease in fuel energy as a result of oxygenate use is in the 2% to 3% range when compared to gasoline. This corresponds to 0.5 to 0.8 miles per gallon for a car that averages 27 miles per gallon. As can be seen from the works cited below, research in this area indicates that any fuel economy loss experienced as a result of oxygenate use agrees with the theoretical prediction for fuel energy loss. Thus, it is reasonable to conclude that any fuel economy loss experienced with oxygenate use is solely a function of the change in fuel composition and the resulting slight decrease in energy content of the fuel.
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http://tonto.eia.doe.gov/oog/info/twip/twip.asp
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MTBE and ethanol contain about 80 percent and 67 percent, respectively, of the energy in conventional petroleum-based gasoline. The result is that typical oxygenate blending reduces the Btu content of gasoline by two to three percent.
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