During the fall and winter of 1970-1971, the Coordinating Research Council (CRC) contracted with Southwest Research Institute for the latter to conduct a pilot program on 10 pairs of 1971 model cars to compare the influence of leaded and unleaded fuels on octane requirement increase (ORI). The cars were operated for 8,000-10,000 miles in the San Antonio area using a prescribed city-suburban driving schedule. One car of each pair was run on a commercially available leaded gasoline of 93 Research octane number (ON) and the other on a specially prepared unleaded fuel of similar quality. Differences in ORI for the two fuels ranged from 0-3.5 ON within car pairs. The average ORI of 4.0 ON for leaded fuel cars and 4.6 ON for unleaded fuel cars was not significantly different above the 80% confidence level.