I know in my cobra from engine to rear tires there is about 15% power loss on dyno. Is it less in s gt40 because the trans is connected directly to tires? What is the average power loss from engine to tires? I have a CAV and going with one of the big boy $15k transaxles.
It's all about the dyno. They are all different, or calibrated differently. There is no answer to your question generally. You would need to know the crank HP and then subtract the wheel power to come up with a "as tested" figure that would not translate to any other data set (different dyno's).
My opinion is that it is not a % figure. Think of a fuel dragster making +7000HP. It is driving the power train with 7000-10000 Hp. The drive train is not much different than any other configuration and could be argued that it is MORE efficient (less power consumption).
It is not using 2500HP up. (25% of 10000HP). That much heat would melt the whole thing in seconds. 2500 X 700ish watts = 1.85 Mega watts!!!!!
My guess is that the average power train, final drive, CV joint's and transmission, etc. is consuming about 50-80ish HP, more or less.
But in the end there isn't really any way to know without back to back testing, the results of which I have never seen in print.
This "how much" question is fun...………...but it really doesn't mean much.