Gearing doesn't much effect the torque, use lower voltage if plan to have mercy on your tranny.
Actually, gearing does have quite an effect on torque. If the motor has say 0.3 fl-lbs of torque at the shaft for a given amperage, gearing down 20:1 increases the gearing factor, so it's 6ft-lbs of torque. Reducing the gearing (increasing the ratio) increases torque at the wheels.