The air-fuel ratio is the mass ratio of air to fuel (gasoline) drawn into the cylinder during engine intake stroke. From the relationship between the air-fuel ratio, engine torque and fuel consumption rate shown below, it is understood that there is no air-fuel ratio in which both torque and fuel consumption can be improved. Moreover, the air-fuel ratio needs to be changed according to the condition of the engine.

Theoretical air-fuel ratio (1:14.7)
The theoretical air fuel ratio indicates the theoretically calculated quantity of air required for complete combustion of gasoline. 14.7 parts of air are required for each part of gasoline and this theoretical air fuel ratio is the most ideal one with engine torque, fuel consumption and exhaust gas countermeasures taken into account.

Running condition and air-fuel ratio
As explained above, the theoretical air-fuel ratio is the most ideal one. However, the actual air-fuel ratio required by the engine varies depending on engine coolant temperature and load. Thus, the air-fuel ratio is altered by the ECU according to the running condition to satisfy the engine's requirements. In general, while idling or running with medium load, the air-fuel ratio is close to the theoretical air fuel ratio but when starting or running with high load, the air-fuel ratio is richer than the theoretical air fuel ratio.