I am a mileage geek. My car gets over 55mpg.
I am also a mountain biker, but alas, only 18 speeds and I admit I don't use them all. I don't even know how I would number them 1st through 18th! (Without sitting down with a calculator and counting teeth).
I have to motorcycles:
The Yamaha XV 250 for comuting. Stock, it came with a 16/45 toothed front/rear sprockets. 1st gear was so low I didn't use it. In 5th at 55+mph the mirror images was so blurry from vibration ... how blurry were they? ... They were so blurry I couldn't tell the difference between a car and a truck! The gearing seemed more appropriate for a dirt bike of which I've had many.
I swapped the 16/45 for 17/38 and it is a different motorcycle, now it feels more like a road bike. 1st gear useful. At the same rpm as the stock at 50mph, I am now cruising at 67mph. Sure I have to downshift sometimes, for passing or long steep hills, but I like shifting. If I didn't like to shift, I'd have gotten a scooter.
A side benefit: stock 16/45 - 79mpg; with the 17/38 - 93mpg. Woo hoo!
My Sunday-go-to-meeting/date bike is a Yamaha XVS 650, shaft drive. I'd love to cruise at lower rpms on the highway. It gets 62mph, as is, (without date).
What I would like to hear in this discussion is not why they are geared the way they are, nor whether it is a good idea or not to change the ratios, but how would anowner make such changes to various bikes if the owner chose to do so. With specific examples, and the results both qualitative (mpg, rpm to mph, etc) and qualitative (runs smoother at speed, too sluggish, etc).
My $0.02?
The Yamaha XV 250 should come stock with the 17/38. It's not a dirt bike. It's not a crotch-rocket. Comfort at cruising speeds is much improved, MPG is significantly improved (for those who care).
Caveats - Less power, shifting will be required to maintain speed on long steep hills on the highway. 5th behaves like a true overdrive - can't just "throttle-up", like you can on bigger displacement bikes. Me, I don't mind shifting, what are 2nd, 3rd, and 4th for on a road bike? Just getting from 1st to 5th?