If your engine takes the garden variety countershaft sprockets, then I'd try a 17T, if it will fit...and it should. Some engines can accomodate up to 18T. The beauty of swapping-out countershaft sprockets is the relative ease compared to the P.I.T.A. of changing wheel sprockets. There is a small price for this relative convenience, the ratio is affected more by a 1 tooth change on the CS than the wheel. If possible, you determine the range by testing CS sprockets, then going up, or down, one tooth an the wheel sprocket to fine-tune the combo. If you have one of the late-model 124/125 motors, it will cost less to buy wheels sprockets and they're easier to source.
In a perfect world I'd go with whatever wheel sprocket gave the best overall gearing with 16T on the CS. That way, there's the option of running 17T for extended high-speed road cruising and dropping down to 15T for trails/climbing all without having to adjust chain length or spend more than 5 minutes changing-out a sprocket.
In the real world, compromises have to be made. There are three schools of thought when it comes to gearing: go for acceleration without regard to top speed; slightly over-gear for relaxed cruising and a bit leftover for the odd banzai blast downhill, using fourth gear as on overdrive; split the difference, which usually means gearing for maximum top speed, i.e. top speed exactly at the rpm the engine makes peak hp. Each scenario has its pros & cons. You have to decide what you prefer.
Judging by your description, you have a low enough first gear to handle quite a heavy sprocket combo. The Honda Nice trans ratios are more closely spaced than what you appear to have; go too large on the CS relative to the wheel and the clutch must be feathered a bit on takeoff. If your first gear is a granny ratio, then you have a good shot at testing out some fairly outlandish sprocket combos. If you go way over the top, at least you'll still be able to takeoff normally and ride back home in third. What you'll find is that once you're on the ragged edge of the engine's powerband, one tooth on the CS or 2 teeth on the wheel will make the difference between "just about right" and overgeared...the bike will slow on the slightest grades and into headwinds, top speed may even decrease.
I take it that you're looking for optimal road gearing and that means finding the upper limit of mph/1000rpm above which the engine begins to lug. Without knowing the gearing and power stats of your motor, you'll have to go by trial & error, plus seat-of-the-pants feel. Every engine has an rpm range that just "feels right" for extended riding. Above this, the engine begins to sound and feel "busy". As a rule of thumb, the upper end of this range is usually just below the rpm at which peak hp occurs which, coincidentally, is about 85% of max rpm. With my stock 110, I know that peak hp occurs at about 8000rpm and the engine pulls well to about 9200. It has enough power to go mid 60s and I like cruising along in the 50-55mph range. Gearing for 7.2mph/1000 rpm (17/31) puts engine rpm between 7000-7500rpm at those speeds, at which the engine is glassy smooth (the rearview mirror is rock steady). Above 8000rpm, the engine begins to buzz a little and loses the relaxed feel. Even though the bike will go along at fairly steady 60mph, 57mph is the upper edge with this gearing. Increasing the gearing to 7.5mph/1000rpm(17/30), ups the ante to a steady 60mph, but a) there aren't many places I ride where this is even remotely legal/necessary and b) uphill performance gets a bit soggy, especially when riding two-up. So, I've compromised and stayed with 17/31 which is low enough to allow easy trail riding in first and second (first can be wound to about 20mph) and ascending long grades at 50mph, or better, even with two aboard. I don't really care all that much about top speed, just having enough power in reserve to deal with hills while taking the wife along, without having to spin the engine any faster than necessary.
It's a lot to digest, but that's the scientific approach...in painstaking detail. Trail & error will get you there, too. It just takes more time & testing.
and hopefully will get one after I sell my CR-80R
