Octane requirements of 55 cars, representing five Ford engine families, were determined every 2000 miles during a deposit accumulation schedule consisting of city, suburban and highway operations. Five cars from each engine family were operated on certification durability fuel to establish the ORI variation within and between families. Additional cars and fuels were used to investigate the effects of lead content, a detergent additive, and higher boiling fuel components on ORI.The data obtained show that among engine families there were characteristic differences in ORI and that the initial ORI build up occurred more rapidly when the investigated detergent additive was used. The fuel with reduced tail-end volatility was found to increase ORI. ORI with leaded versus unleaded fuels was found to be greater with unleaded fuels but with wide variation between the engine families studied.