Hmmm...... I'll agree with you to a point.... My Farmall M tops out little over 20 mph in 5th gear. If I had a 6th gear to drop into, it could easily go a little faster. I think we can all agree that aerodynamics plays a very small part in the top speed of my old M. It has enough power to go faster, but not the gearing. To this point, I'm with you.... but we separate paths when you keep maintaining that horsepower is not a limiting factor on speed.
Torque is twisting effort. As the engine shaft provides twisting effort, work is done. Power is the RATE at which work is done. Any engine or other prime mover has a limited, measurable, and pretty much constant amount of torque at a given RPM.
Propelling a car, tractor, truck, or bicycle takes a certain amount of power. That power varies, with higher speed needing more power. Why? Because power is a RATE of doing work; horsepower is a fixed ratio of work output or rate of doing work as fixed by James Watt, in an effort to equate the power of his steam engine to that of a horse.
All machines have friction. Friction in the drive train, tires on the road, etc. You can reduce this friction to some extent, but it's always there. A train, running on steel rails, has rolling friction. To eliiminate this, engineers have been playing with magnetic levitation for trains. Unless your car or truck, or my old M is converted to magnetic levitation, rolling friction must be overcome. That is work. Work takes power. Moving faster takes MORE power, as power is a measure of the RATE at which work is done.
Aerodynamics comes into play as speed increases. Not much of a problem on my Farmall M, but it is a noticeable factor at highway speed. As speed increases, aerodynamic drag increases even faster. Generally, on most shapes, if you increase the speed, the drag goes up as the square of the speed. Double the speed from 50 mph to 100 mph, and the drag will likely be close to FOUR times what it was at 50 mph. Going 150 could result in needing NINE times the power to overcome the drag at 50 mph. To keep this under control at high speed, you need to start with a reasonably aerodynamic shape.
Back to friction for a moment....all gears, bearings, etc, have friction. I don't doubt that you can crush an aluminum can with one finger using enough gear resuction. Using even more reduction, you could theoretically crush that can with the power generated by my watch spring. It might take two or three weeks, but it could do it......UNLESS the friction in the gearing could not be overcome by the power source, in this case the watch spring. If the watch spring was capable of crushing the can, it would do it extremely slowly, as it doesn't have much POWER, which again is the RATE at which WORK (crushing the can) is done.
As I recall, this whole theme got started with a discussion about how fast a car could go. The faster a car goes, the more work is being done, due in part to aerodynamics. The faster a car goes, the more POWER it takes, because power is the rate, or speed of doing work. Could my 24 hp Briggs Intek lawmower engine do the field work of my Farmall M? Perhaps, if it had enough gears to go VERY SLOWLY. Could it drive my 2010 Siverado 4wd at highway speed? Probably not much more than 30mph, even if it had 100 gears, because there isn't enough POWER to do the work at the speed I want to go.
I don't know how many horsepower it takes to shove a '55 Chevy 150 mph. I don't know how many hp it takes to run a car nearly 200 mph. But I know that there is measureable amount of power needed. Going faster takes more power (and usually lots more $$) in the process! Could an old Chevy hit 150? Maybe....with enough modifications. Could a pickup truck, or service truck, or semi, or Greyhound bus go 100 mph? In many cases, the answer is probably yes, but NOT on the POWER of a lawnmower engine, regardles of how many gears it has.
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