<P> A gasoline - fueled reciprocating engine requires fuel of sufficient octane rating to prevent uncontrolled combustion known as engine knocking (knock or ping). Antiknock agents allow the use of higher compression ratios for greater efficiency and peak power . Adding varying amounts of additives like low percentage TEL or high percentage ethanol to gasoline, allowed easy, inexpensive control of octane ratings . TEL offered the business advantage of being commercially profitable because it could be patented . Aviation spirits with TEL used in WWII reached 150 octane to enable supercharged engines such as the Rolls - Royce Merlin and Griffon to reach high horse power ratings at altitude . In military aviation, TEL manipulation allowed a range of different fuels to be tailored for particular flight conditions . </P> <P> In 1935 the licence to produce TEL was given to I.G. Farben enabling the newly formed German Luftwaffe to use high - octane gasoline . A company, Ethyl GmbH, was formed that produced TEL at two sites in Germany with a government contract from 10 June 1936 . </P> <P> In most industrialized countries, a phaseout of TEL from road vehicle fuels was completed by the early 2000s because of concerns over air and soil lead levels and the accumulative neurotoxicity of lead . The use of catalytic converters, mandated in the US for 1975 and newer model - year cars to meet tighter emissions regulations, started a gradual phase - out of leaded gasoline in the US . 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 MMT as well as oxygenates including MTBE, TAME, and ETBE . </P> <P> Lead - replacement additives were scientifically tested and some were approved by the Federation of British Historic Vehicle Clubs at the UK's Motor Industry Research Association in 1999 . </P>

When did the us stop using leaded gas