What type of DC motor it is?
Series, shunt, compound, permanent magnet?
Regardless, your measured values appear to be quite high for a DC motor.
According to the DC motor's equation:
Terminal voltage: V = LδI/δt + Ia*Ra + kb*ω
and to a first order approximation Torque: τ = kt*I
and you solve for speed as a function of torque, keeping the terminal voltage constant, you will find that changes in the load torque will cause a very significant change in the motor speed, and depending on the moment of inertia, the system could even become unstable.