Gearing and speed are always common topics on any orv powersport board. In an effort to answer some broad questions I offer this post.
Stock Top Speed:
There are a lot of answers out there, but most are in the range of 68-77 on a flat patch of good ground. FME it's in the low 70's. Anyone claiming 80+ is either modified, going down a huge hill with a tail wind, or blowing smoke. Anything lower than 65 either has modifications (shorter gearing/smaller tires) or there's something wrong with it.
For most of us 70 is more than fast enough. At those speeds and higher the tires, twitchy handling and wind resistance start making things significantly less safe. If you're one of these people it's a good idea to shorten up the gearing to accellerate faster.
If you have a reason to go faster or a need to lower the rpms in top gear for a cruising speed then it's time to look into taller gearing.
The 660 accepts front sprockets of 12-15 teeth with the oem shroud, 16 tooth will work without it. The rear sprockets can be sized from 39 teeth on up, with your ground clearance most likely being the limiting factor. Anything smaller than 39 and the chain will start to ride on the sprocket hub mounts instead of the sprocket.
For an idea on what sprockets do to top speed on a stock bike, I offer these calculators. In stock trim the 660 revs somewhere in the 7k-8k rpm range, but more on that in a minute.
Sprocket/Top Speed Calcuators
(Thanks to SandSquid for all his work on this one)
This is all keeping in mind that your bike is stock, meaning that it can only rev so high and only has enough hp to pull a certain gearing. But when you can rev higher the bike will go faster.
You'll notice in pipe comparison dynos that most aftermarket pipes not only increase the hp but also increase the peak rpm. Many of the modifications that you add to your machine will have a similar effect, including when you free up air intake. This also means that your top speed will increase, assuming as we will that you're not limited by wind resistance. So if you were able to go 70mph stock adding more hp might make that same bike go 75mph now. The more hp and higher revs you can get out of the bike, the faster the bike will go on that gearing.
But this also has an impact on gearing. If you only need to go as fast as you did stock and you modified the bike to reach higher rpms and therefore a higher top speed then you can change your gearing to keep that stock speed and get there faster at the same time.
The following numbers are for explanation purposes only. For instance, say your bike does 75 stock. You do some mods, now your bike goes 80. We all know that lower gearing (higher ratios) means less top speed, but we also know that it means increased accelleration. If you go from 13/40 down to 12/40 on your modified bike you'll have the benefit of greater accelleration and because your bike can rev higher than it did stock you can maintain the 75mph top end at the same time.
My 660 is a good example for me. Stock it did somewhere in the low 70's. I've made some modifications and now running 12/42 gearing and 20" tires my bike has the ability to rev high enough to exceed 78mph. My top speed hasn't gone up all that much, but with the shorter gearing I'm able to reach the original top speed significantly faster that I did when it was stock. And honestly it's still a little too fast on top so the 44 tooth is ready to go on for next year.
I've explained, at least in part, this topic as best as I understand it. If there are any discrepancies or questions please let me know. Hopefully it's able to clear some things up.