Transmission losses MUST be relative to the amount of power transmitted.
Coastdown test is all very well for "fixed" losses, but do not take into account "dynamic" losses.

I'll try to give an example...

Take one clutch.
Torque rating is calculated around: Co-efficient of friction; contact area (both sides), CLAMPING PRESSURE.

Your transmission will have a certain co-efficient of friction per gear.
What can vary however is torque. Equate that to clamping pressure.

Take one car. with a certain engine/transmission/wheel bearings/ etc.
Increase the torque level of the engine, and the losses will go up.

Think of it like this: lightly put your hands together. Now start rubbing them backwards and forwards.
Makes them warm yes? Now push your hands together harder. Rub them backwards and forwards at the same speed as before.
Fuck me that's hot!

Now, the co-efficient of friction didn't change, you used the same pair of hands as before. What changed was apply pressure.

This is the same as transmission losses, as you increase torque.

We can reduce these losses to some degree by altering your co-efficients of friction through your transmision components, but, on a given transmission train; the more torque, the more the transmission losses.

I've also got a feeling that a graph of this would not have a straight line...