Non-linear torque vs speed response for DC motor
The Current required by the driving motor is not necessarily a linear function of speed.

The current into the driving motor depends on the torque required of it as well as a other factors. The torque depends upon its own friction as well as the output torque to drive the load.  There will also be current required to account for electrical losses.  These other terms may not be linear with speed.

To determine the torque being absorbed by the generator first plot a curve of the motor current without the generator attached - this will give you the current required just to spin the motor.  Then repeat the experiment when driving the generator.  The difference in current should be a function of the torque to drive the generator.

You should perform the test with various loads on the generator at each speed to get a complete characteristic.

Subtract the plot with no-load from the ones with a load. That will represent the additional current required to drive the generator. Multiply that current by your torque constant to get the actual torque required by the generator.