I'm curious as to where the 3/5 rule comes from and how/why this figure came to be. I've never heard of this. I was taught that "horsepower" is a fixed value. It does not change according to the source of the power. There may be differences in efficiency, losses through gears & transmissions, etc., but 1 HP = 550 foot-pounds = 745 watts no matter where it comes from. It's been a long time since I've been in technical college, but I'm certain the laws of physics have not changed.