Torque is not really a controller function, but it is directly related to the current flowing in the motor. The mechanical torque is proportional to the motor current and the relationship is defined by the motor’s design, details will be in the motor specification.
However, motor torque is not the output torque at the wheel. The final drive torque is the motor torque multiplied by the drive gear ratio. But the final speed is the motor speed divided by the drive gear ratio. So it follows that if you increase speed – you reduce final drive torque and vice versa. For your machine, you need to arrive at the correct compromise.
We have emphasised drive gear ratio. This is not the same as the gear ratio you have but also includes the diameter of the road wheels. Imagine a motor rotating at 3000 r.p.m. with a gearbox ratio of 10:1. The output speed of the gearbox will naturally be 300 r.p.m. This is the shaft that is rotating the wheels. So the wheels will rotate at 300 rpm. So the machine will move at 300 times the wheel circumference per minute. Final drive gear ratio is therefore directly affected by the wheel diameter.
If you have too much torque – then when you meet resistance (such as another robot) something has to give. It is going to be traction which is lost: you (or the other robot) will slip. So the torque you need is entirely down to how well your robot grips the ground. No point in having immense torque if your wheels slip.
If your torque is too low, then in a head-to-head shove, you will loose. But low torque implies high speed. High speed does two things: it makes your robot more difficult to steer, but it also means that the kinetic energy in your robot is high. So if you are going to employ high speed collision techniques, you need speed, not torque.
The art of making a successful machine is in getting the trade off correct!