What primarily determines the fuel efficiency of an aircraft at a given weight and speed?

The primary factor that determines the fuel efficiency of an aircraft at a given weight and speed is altitude. As an aircraft climbs to higher altitudes, the air becomes less dense, which significantly reduces aerodynamic drag. This decrease in drag means that the engines can operate more efficiently, allowing the aircraft to burn less fuel for the same speed.

Higher altitudes also enable engines to operate in a more optimal range, where they can achieve better performance and fuel consumption rates. In contrast, flying at lower altitudes where the air is denser increases drag and impacts fuel efficiency negatively.

While engine type, cost index, and weight distribution do influence fuel efficiency, they are not the primary determinants when considering the effects of altitude on overall performance during flight. Thus, altitude stands out as the crucial factor in optimizing fuel efficiency for an aircraft.