What do you mean by the max power of the magnetic field?

My understanding is that to maximise torque delivered to the wheels one needs to maximise the torque obtained from the motor. By running it at a high RPM and gearing further down, you're effectively increasing torque through the gearing. So a motor spinning at 60k RPM geared down to 3k RPM will result in more torque at the wheels than the same motor geared from 30k to 3k.