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Most of today's commercially available vehicles can run on blends of E10, a blend of 10 percent ethanol and 80 percent gasoline, or lower. E10 is the most common low concentration blend. Many areas of the country mandate its use as a replacement for MTBE.

MTBE is made primarily from natural gas or petroleum products. It has been used in U.S. gasoline at low levels since 1979 to replace lead as an octane enhancer, helping to prevent engine "knocking." Since 1992, MTBE has been used at higher concentrations to fulfill the oxygenate requirements required by the Clean Air Act Amendments of 1990 (CAA). Oxygen helps gasoline burn more completely, reducing harmful tailpipe emissions from motor vehicles. The CAA required regions with poor air quality (nonattainment) to use gasoline reformulated with an oxygenate. Most refiners chose MTBE over other oxygenates for financial reasons and for its blending characteristics.

After MTBE was proven to be a carcinogenic groundwater pollutant, many states banned MTBE, which accelerated the use of ethanol as the only economically feasible fuel additive for states with air quality problems. The Energy Policy Act of 2005 further accelerated the demise of MTBE in that gasoline marketers were no longer required to use an oxygenate, but they also did not receive the MTBE liability protection that they had petitioned for. This put refiners at a liability risk if they continued to use MTBE after an oxygenate was no longer required by law. Predictably, the demand for ethanol increased at unprecedented rates as most refiners replaced MTBE with ethanol.

Even though refiners were not obligated to use an oxygenate after May 6, 2006, most of them switched from MTBE to ethanol primarily for these reasons:

	There still exists EPA emissions control requirements. Adding oxygen to gasoline allows for more complete combustion of the fuel, which reduces exhaust emissions of CO (carbon monoxide), which can result in lower levels of toxic, ozone-forming pollutants and greenhouse gases, especially if higher-level ethanol blends are used.
	Ethanol has the advantages of MTBE without the same dangers, and it contains 35 percent oxygen by weight, twice the oxygen content of MTBE. Ethanol is the highest-octane fuel in the market. Because it doesn't leave gummy deposits, it helps to keep the vehicle's fuel system clean for optimal performance.
	Ethanol extends gasoline supplies. Because its production is encouraged by a federal tax credit of 51 cents per gallon, ethanol is economically attractive. Without this credit, it would not be as competitive.

U.S. Ethanol Consumption (millions gallons) 1993-2025
	DOE's Energy Information Administration estimates that approximately 130,000 barrels per day of ethanol will be needed to meet the demand created by refiner decisions to replace MTBE, and U.S. ethanol production is currently running at capacity as refineries reduce MTBE usage. See this EPA fact sheet.
Ethanol and MTBE Consumption, 1992-2004	This DOE Energy Information Administration graph compares ethanol and MTBE consumption in the transportation sector during 1992-2004.
See Eliminating MTBE in Gasoline in 2006, a February 2006 study by the Energy Information Administration.

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