Drag racing is another story altogether... all about 0-10mph, the first 60 feet, probably less. I'd compare that to gearing for swap meet riding, where top speeds are nowhere near what the motor can pull and being able to all but drop the clutch from a dead dig translates into huge pulling power/rapid acceleration out of the hole.
Road riding over longer distances, like 100+ miles in an afternoon, is the other end of the spectrum. That takes some saddle time and testing to truly dial-in the gearing. If you never ride more than, say, 30 miles at a stretch/50 miles in a day, it really doesn't matter that much. Go out for 3,4,5 hours over the road and you'll not only notice things for the first time, some otherwise minor things may become downright annoying. For example, every single has a "sweet spot" in the revband...as well as a rough spot, usually only a few hundred rpm wide. If it happens to fall just at the speed where you tend to cruise along, being unable to see clear images in the mirrors, plus the "buzziness", may grate on your nerves after a time. It's amazing just how much difference a slight change in cruising rpm can make. It also depends upon how discriminating a rider is in their mechanical preferences. In case it's not crystal clear, already, I border on obsessive when it comes to carburetion & gearing; IMHO, those are like "free horsepower"...or, more accurately, horsepower you needlessly give away when those parameters are less than optimal.
I disagree, partially, about the "one and done" gearing solution. The explanation is kinda longwinded. For the sake of brevity (and sanity) I'll start with the extreme...the absolute limit of how tall you can go, before it becomes impractical. It is possible, with some tunes in this displacement range, to gear tall enough that a bike can pull its maximum speed on grades, in 3rd gear. That'd deliver maximum "snap" at, say, 50-55mph. The reason it will almost certainly be impractical is that the clutch would have to be feathered, from a dead stop; from experience, 4th gear would almost certainly be too tall, as well. the ratio spread of these transmissions are surprisingly well matched to the engine, almost regardless of displacement. This is more theory than practical instruction but, as with carb jetting & sizing, knowing the theory can save a lot of time. All of these engines, from 49cc stockers to the 125cc strokers (the largest displacement that is still practical with 49cc cases) are horsepower tunes (more hp than torque). Thus, they usually work better when slightly under-geared...and perform weakly (in top gear) when slightly over-geared. At the other extreme, cruising-through-top end, it comes down to a choice between gearing biased toward minimum rpm/mph and pulling power...whether to use 3rd for steep grades because 4th is too tall, or gear low enough that 4th can be used...and having the motor spin more revs per mile than it has to, everywhere else.
Okay, all that windage having been committed to text, here's a bit of applied fine-tuning, learned the hard way...lots of years and miles. Terrain varies greatly, by locale and it does affect optimal gearing but, not by as much as you might expect. Generally speaking, 3rd gear can take you to within ~15mph of top speed,(engine redlined in 3rd), but with far greater pulling power. And 4th gear has an overall range that's ~40mph wide. That's a lot of overlap. And it's this overlap range that gets overlooked, too often. A 65mph bike that can pull 50mph up a <relatively speaking> steep grade is probably going to be able to do that in 3rd or 4th...a choice between higher revs (using 3rd) and more throttle (using 4th). If the grade + wind conditions drag speeds below that point, a downshift will be required, regardless...and it's basically impossible to over-gear to a point where 3rd exceeds available torque. Within these conditions, there's nothing quantifiable to be gained by under-gearing. This is where the process gets interesting...gearing to allow "surfing the horsepower".
There are some conditions...combined effects of grade, wind, ambient air temp...where the bike may only reach its speed potential if the rpm can be maintained in 4th gear. IOW, slow down too much and you won't be able to downshift/rev-out/upshift your way back, until you're riding over more level terrain. I think of it as being like "crack the whip"...going into a grade at a high enough speed and the bike will maintain it, maybe even gain a few mph along the way, which it couldn't do any other way. The same applies to top speed runs. I've at least matched the my peak speed numbers while ascending slight grades; the key element was reaching a certain entry speed...then the motor just kept adding mph all way the uphill. This is where finding the torque limit really pays a dividend. Find the balance point and general -over-the-road performance will be acceptably strong and at minimum revs-per-mile. What makes this tricky is the fact that every engines power curve has small peaks & dips all the way to redline. Only way to figure this out, if one lacks access to a chassis dyno, is the seat-of-the-pants dyno. It takes time to accurately assess power output, in real time and in real world riding.
Anyway, apologies for the lengthy dissertation. I figure that you've come this far with build & its tuning, you may as well have an overview of gearing. With these little machines, another 2-3 mph can't hurt, for sure. More than that, however, is the overall feel while at the controls and that's even harder to describe.
Maybe even 2 to 3 mph increase in top speed.
16 is going back on soon.