Interesting stuff. It takes all aspects of the equation to accomplish our objective of moving our rig/ load down the road, and up the hill but yes, it all begins with HP. Add to that the trans/rear end differential gears, tires etc turning and you produce torque. This all works together to achieve what the vehicle is designed for. How does a 40' gasser motorhome with a 400 HP engine make it up a hill? Gears. Probably in excess of 5:1, but as turtle n peeps pointed out, the duty cycle is pretty bad. How many of those big old gasser MH engines went over 200K miles? Put those gears in my truck and I could match the diesel boys for pulling power, but the life of my engine would be short. The motorhome can't drop it's load and run around empty like my truck can. So the engineers that designed my truck knew I can't be commuting every day on the freeway with 5.30:1 gears and design the entire system to do that, and pull my trailer occasionally quite well but yes, HP must come first. Without the engine first turning fuel into force ( measured in HP), you have nothing.
