In recent years, several programs have been conducted at the Ethyl Research Laboratories in Detroit, some in cooperation with a major oil company, to measure the value of lead antiknocks in fuels for European cars. These heretofore unpublished studies show: 1.A positive lead road bonus. 2.Tetramethyllead (TML) is more effective than tetraethyllead (TEL). 3.The lead road bonus increases with engine operating severity. The lead road bonus is defined in broad terms as a combination of three effects - lead-in-fuel, fuel sensitivity, and engine deposits. The engine-deposit effect is not of major significance in European cars.Much of the confusion over the value of lead antiknocks in European cars and fuels in recent years can be attributed to improper definition of the lead road bonus, at times coupled with inappropriate rating conditions and limitations in fuel design.The importance of these studies is significant in view of the trend toward smaller cars.