The only solution that I can possibly think of is the fact that in order to handle the hgiher amounts of poer created by the V8 those transmissions have to be more sturdy. Thus creating more drag.
V8 has 300hp withbeefy tranny so it gets 15% driveline loss = 255rwhp (loss of 45hp)
I4 has 100hp with wimpy tranny so gets 15% driveline loss = 85rwhp (Loss of 15hp)
This way the percentage stays the same, but hte actual amount of power needed changes, Due to the stronger gears/bigger flywheel/heavier duty driveshaft.
That is the only thing I can think of that would cause us to use a percentage. I will ask a couple of the car Gurus at work today and see what answers I can come up with.
