is any of several lead compounds used to improve octane number in a gasoline. The best known is tetraethyl lead (TEL), Pb (C₂H₅)₄. Another is tetramethyl lead (TML), Pb (CH₃)₄. Other compounds have varying proportions of methyl radicals (CH₃) and ethyl radicals (C₂H₅). Use of lead compounds in motor gasoline is being phased out for environmental reasons. Beginning with the 1980-model year, all new U.S. and foreign-made cars sold in the U.S. required unleaded gasoline.