Percolation is a problem resulting from vapor formation in the fuel system of automobiles as well as vapor lock. Percolation has recently become very serious in Japan. Road tests were carried out using four Japanese cars and one American car to determine the percolation performance of fuels. The percolation performance of fuels was correlative with 30% and 50% evaporated temperature, representing mid-range volatility, however, it was not correlative with Reid vapor pressure and 10% evaporated temperature, representing front end volatility. The influence of cars and test conditions on percolation was also studied.