E10, once commonly known as gasohol, is a fuel mixture of 10 percent ethanol and 90 percent gasoline that can be used in the internal combustion engine of most modern automobiles and light-duty vehicles without need for any modification on the engine or fuel system.
E10 blends are typically rated as 2 to 3 octane higher than regular gasoline and are approved for use in all new US automobiles, and are mandated in some areas for emissions and other reasons. The E10 blend and lower ethanol content mixtures have been used in several countries, and its use has been primarily driven by the several world energy crises that have taken place since the 1973 oil crisis.
Similar blends include E5 and E7. These concentrations are generally safe for recent engines that run on pure gasoline. Some regions and municipalities mandate that the locally-sold fuels contain limited amounts of ethanol. One way to measure alternative fuels in the US is the "gasoline-equivalent gallons" (GEG). In 2002, the U.S. used as fuel an amount of ethanol equal to 137 petajoules (PJ), the energy of 1.13 billion US gallons (4.28 GL) of gasoline. This was less than 1% of the total fuel used that year.
E10 and other blends of ethanol are considered to be useful in decreasing US dependence on foreign oil, and can reduce carbon monoxide (CO) emissions by 20 to 30% under the right conditions.  Although E10 does decrease emissions of CO and green house gases such as CO2 by an estimated 2% over regular gasoline it can cause increases in evaporative emissions and some pollutants depending on factors like the age of the vehicle and weather conditions. According to the Philippine Department of Energy, the use of not more than a 10% ethanol-gasoline mixture is not harmful to cars' fuel systems. On October 27, 2006, though, the Federal Aviation Administration published their Special Airworthiness Information Bulletin - Automobile gasoline containing alcohol (Ethanol or Methanol) is not allowed to be used in aircraft.
See also Ethanol.