In 1962 and 1964 the Coordinating Research Council (CRC) evaluated the vapor locking characteristics of about 40 selected new model cars during centralized programs at Yuma, Arizona. Volatility tolerances were determined during acceleration following a hot soak at approximately 100 F ambient using two reference fuel series with diverse front-end volatilities. Fifty per cent of the 1962 model cars tested had a limiting vapor pressure equal to or less than 8.4 lb with the high volatility fuels and 9.7 lb with the low volatility fuels. Comparable values in the 1964 program were 8.9 lb for the high and 10.4 lb for the low volatility fuels. Analytical results obtained on vehicle tank samples for the first time during 1964 showed that limiting temperature at a given vapor-to-liquid ratio also was a useful parameter in defining car volatility tolerances.