I'm no math wiz, nor am I that savvy about physics and engineering, but if my love for motorcycles is helping me understand the concept behind swapping gears and sprockets; it's simple: by lowering the RPMs at a given MPH, it essentially reduces stress on the engine by better preventing over revving it. Additionally based on 2M's comment about his Chevy, lower RPM at a higher MPH allows more headroom for faster MPH at a higher RPMs.
Do I have the right idea, or how exactly does this all work? Why are we obsessed with keeping our RPM's lower at certain speeds?
Gearing is not only about comfortable cruising at a preferred speed on a flat road. It is always a compromise that has to allow for steep hills, heavy riders, passengers and luggage, strong headwind, acceleration, fuel economy and so on...
Now the comparison beteween a Chevy SB (which mostly is in a really low state of tune when stock) is "apples and oranges"
For any vehicle, the top speed will be the speed and rpm where the total drag+rolling resistance is equal to the power at that given rpm. If this occurs at peak HP rpm, the ultimate top speed has been reached. Raising the gearing further (smaller rear sprocket) will result in a LOWER top speed. Lowering the gearing (larger rear sprocket) will let the engine rev past its power peak, but at a lower speed than the ultimate top speed. Any change in the engines power curve, max HP, the aerodynamic drag of bike+rider, rolling resistance will require a corresponding change in gearing in order to reach the ultimate top speed for that particular condition.
I have a feeling that many here on the forum has a bit of what I would call "Harley Envy", and want the XS to chug along at too low rpm. Instead it would be better to focus on reducing vibration by means of lighter pistons and rods, crankshaft balancing or re-phasing, good carburetor synchronization, correct ignition timing, and so on. I am pretty sure the Yamaha engineers NEVER intended the XS to run with a 30/18 sprocket set. If an engine gives best peak power at say 7000 rpm, it makes little sense with a gearing that will not let it rev past 5500 in top gear.
-Just my 2 (mechanical engineering) cents