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. Whereas the lowest octane rating is 87 AKI (antiknock index) for regular gasoline at most service stations throughout the U.S., 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 (1660 m elevation). A chassis dynamometer test procedure was developed to measure absorbed wheel power at transient and stabilized full power operation. Five vehicles were tested using 85 and 87 AKI fuels. Turbocharging, natural aspiration, gasoline direct injection, and port fuel injection technologies were 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 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 earlier onset of knock when operated on 85 AKI fuel, and used different engine control settings for knock avoidance.