The effects of selected fuel additives, engine operating conditions, and driving modes on ORI have been investigated for the 1975 Ford 400 CID engine. A twenty-seven vehicle program was designed with three vehicles per variable. Reduced coolant temperatures significantly lowered ORI. Methanol appears to reduce ORI. Variables which do not produce strong trends are increased air-fuel ratio, a driving cycle with wide open throttle accelerations, a fuel with a carrier oil additive, and heavier fuels.An inverse relationship between initial octane requirements and ORI was observed.