Come up with a value for the torque requirement (for example the load that your robot presents - Pull 130 pounds of potatoes on a mock up of your wheel system, if necessary).
You can see by the figures for free speed and the geared down speed (multiplied back up to the motor speed input to the gears) how much "friction" the gear box represents.
Then, with the a Speed-Torque curve for the motor in question in front of you, enter on the torque axis, go up to the line, the at the line, move horizontally to the left, and read the speed.
