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In page Tetraethyllead:

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The use of catalytic converters, mandated in the United States for 1975 and later model-year cars to meet tighter emissions regulations, started a gradual phase-out of leaded gasoline in the U.S.[2] The need for TEL was lessened by several advances in automotive engineering and petroleum chemistry. Safer methods for making higher-octane blending stocks such as reformate and iso-octane reduced the need to rely on TEL, as did other antiknock additives of varying toxicity including metallic compounds such as methylcyclopentadienyl manganese tricarbonyl (MMT) as well as oxygenates including methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME), and ethyl tert-butyl ether (ETBE).[citation needed]