What I don't get is, doesn't take X amount of hp to turn a trans, rear end, tire etc.. Shouldn't take a similar amount of power loss at similar rpm (not a percentage), be it a 300 hp engine vs 600 hp engine ?
I wish it was that easy. But it’s not. Friction increases with RPM and MPH.
And like I said before, unless you do a coast down on every vehicle you don’t have a clue what it takes to run the drive train.
There are some exceptions to that. One being if you have a dedicated dyno car. In that case, you are using the same car, same transmission, same rear axle so the percentage will be the same every time.