lead alkyl - 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.