A mixture of petrol (gasoline) and a relatively small proportion of alcohol (i.e. typically ethanol at 5% or 10%) that can be used as an alternative fuel for cars and other vehicles without modification of the engine. The percentage of ethanol in the fuel is denoted by the E number, e.g. E10. Ethanol-based gasohol has a higher octane rating and burns more completely than conventional petrol, thus lowering some harmful emissions. Often the ethanol is obtained as a biofuel by fermentation of agricultural crops or crop residues, (e.g. sugar cane waste). Vehicle engines can also be modified to use mixtures containing greater proportions of ethanol. For example, blends with over 20% ethanol have been used in Brazil since the 1970s, and a mixture of 85% ethanol and 15% petrol, called E85, is available for flexi-fuel vehicles in some countries.