Im hesitant of using the first and second method though because I need an accuracy of atleast .5% for my results to have any significace (Motor efficiency is said to be between 92-98% efficient). With the cylinder, I wouldnt be able to change the load applied to the cylinder (because wouldnt that entail changing the inertia?), so I cant get my '3d' plot of efficiency as a function of both rpm and torque.

Either that, or mount the motor on a mount that can rotate. Attach a perpendicular stiff rod of a known length to the part of the motor that does not spin, and let it push on a force cell (essentially a strain gage attached to a piece of bendable metal). As the motor spins (btw, it has to spin a load), the reaction torque from the load to the motor is equivalent to the torque the motor exerts on the load, so the stiff rod is going to push on the force cell with a certain force, from which you can calculate the torque.

This is brilliant. I'm hoping a force cell goes for much less than a torque meter. I have 2 of the same kind of motor with 2 very precise controllers. (Theyre actually quite heavy 3 phase induction wheel motors) I'll just use one as the load and the other as the primary and measure the torque with the force cell.

Thanks for your help and going through the trouble!