Top gear on most recreational motorcycles is not meant to be the "fastest" gear. Top gear is typically a "cruising" gear....where you maintain a relatively high speed, for long periods. The RPM in this configuration is relatively low. This gives smooth, quiet running and good fuel economy.....
The fastest possible gear would be where the motor would reach the max horsepower rpm...and hit the "drag wall" at the exact same time. If you "streamlined" the motorcycle, then drag would be less..an you would need a slightly higher gear. If you modified the engine for more power, you would then also need a slightly higher gear, or if you modified the engine to make more power at higher rpm you might not need a higher gear....
The gearing on street bikes is designed to be "all around"...that is, good for all occasions and not exceptional at any.
Re-gearing a street bike is a fool's game.....you may improve one area, but you will certainly degrade another...everything is a trade off.
Some like a lower first gear for better "jack rabbit starts".....but come on....do you really do a max hole shot every time you ride, at every red light, and every stop sign? I true hole shot start would require revving up to redline anyway...not practical....the additional down side to lower ratios is that the cruise performance at ahiger speed...the engine is revving high...noisy vibration, poor fuel economy...
Some like higher overall gear ratio...for long, high speed cruising...to run at lower rpm....smoother, quieter, better fuel economy. Downside is sluggish starts.....
Contrary to common belief......the middle gears don't matter....an argument for another time....
Do what you want.....have fun...
Cookie
@Jako has it right.
Think of the two sprockets in combination as a lever (final gear ratio).
You have an object to lift. The weight of the bike an you, wind resistance, and inertia, etc..
You take your fulcrum and that lever which can be longer or shorter (final gear ratio).
You have time (speed).
You have power (your personal strength).
At some point you find your self just hanging off the end of the longest lever. (Archimedes Greek Mythology)
In between the lever lengths you choose there is a sweet spot of work being done and the pace at which your arms are moving.