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