[SpEEdyBL]

The main thing that determines the efficiency of a motor is the Io. That's amount of current the motor will draw at no load. Take an 8xl for example. Io is 3.6 amps. 21 volts @ no load means the motor is using 75.6 watts. This is known as iron loss. That's how many watts are automatically being wasted as heat, since the motor isn't doing any work at no load. The problem is, the 8xl will always be wasting an extra 75 watts of power due to friction in the bearings and inconsistancies in the magnetic field. However, the 10xl which has an Io of 2.3 amps will only waste 59.8 watts of power at 26 volts. So there you have a more efficient motor.

Now, when iron loss equals copper loss, (ohms x amps^2), you get the amp rate at which the motor runs the most efficiently. The fewer amps, the better.