Automakers are designing smaller displacement engines with higher power densities, to improve vehicle fuel economy while continuing to meet customer expectations for power and drivability. The specific power produced by the spark-ignited engine is constrained by knock and fuel octane ratings. Knock can lead to engine component damage and excessive exhaust gas temperatures that may cause thermal deactivation of catalytic convertors. Whereas the lowest octane rating is 87 AKI (anti-knock index) for regular gasoline at most service stations throughout the country, 85 AKI fuel is widely available at higher altitudes especially in the mountain west states. The objective of this study was to explore the effect of gasoline octane rating on the net power produced by modern light duty vehicles at high altitude conditions (1660m elevation). A chassis dynamometer test procedure was developed to measure absorbed wheel power at momentary and stabilized full power operation. Five vehicles were tested using 85 and 87 AKI fuels containing 10% ethanol. Turbocharging, natural aspiration, gasoline direct injection, and port fuel injection technologies were all evaluated using this group of vehicles. All vehicles in the study had some reduction in net power when operated on 85 AKI fuel compared to the 87 AKI fuel. Vehicles with turbocharged engines had a greater net power loss using 85 AKI fuel compared to vehicles with naturally aspirated engines. Engine control system data provided strong evidence that all vehicles were able to detect the onset of knock when operated on the lower octane fuel, and used different engine control settings for knock avoidance.