DC motor efficiency

Hello,
I would like to know if the motor efficiency is changing with respect to applied voltage or it doesn't change. What I do mean. The efficiency=(V*I)/(Torque*velocity) and the power losses= I^2*R. Can I say that
mechanical power ( Torque*speed)=V*I-I^2*R
If so, can I say that the efficiency is not constant with torque and current changing?

Also, the torque is function in current
T=(motor constant*current)=km*I
What about the efficiency? Does the efficiency effect on the motor performance? Can I say
T=efficiency*motor constant*current

Many thanks

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 the torque and velocity want to track with the voltage/current in a linear fashion but the level of saturation in the mag field and the effect of heat on conductance will insure that they cannot track linearly so, as you have surmised, the efficiency will change with voltage/current.