During 1970-1971, 15 organizations participated in a Coordinating Research Council program to compare the influence of unleaded and leaded gasolines on octane requirement increase (ORI). A total of 147 1971 model cars (47 pairs and 53 individual cars) were operated for 8000-20,000 miles using driving schedules prescribed by each participant One car of each pair was run on an unleaded fuel of 91-95 Research octane number (RON) and the other on a leaded fuel of similar quality. For paired cars, the average ORI was 5.8 RON for unleaded and 3.8 RON for leaded fuel cars. The ORI difference of 2.0 RON for paired cars was slightly less than the average ORI difference of 2.5 RON for all 75 unleaded and 48 leaded cars. In a supplemental program, results in 20 cars showed an average ORI difference of only 0.3 RON between unleaded and low-lead fuel cars.