The wide operating range of an air motor makes it probable that a number of different motors could run at the same working point. Where on the torque-speed curve you find the working point affects your choice of motor.
Because it is most efficient to run the motor at the speed produced by maximum power, selecting the motor that produces maximum power as close to the working point as possible will give you the best possible alternative. This motor will also give the lowest air consumption for a given power output.
If stable speed of the air motor is important you should avoid working with speeds below the point of maximum power. This assures that there is some “reserve power” in the case of load increase. It is also a good idea to work closer to the idling speed when there is uncertainty about the torque demand.
Low speed/high torque places a high load on the gears. High speeds affect the lifetime of the vanes.
If a very long lifetime is demanded, a large motor that is restricted or run at a low pressure should be chosen.