[culcune]

The one advantage you might have with the 21/18 size wheels is it might use a different rear sprocket. 'Stretch' stated the term 'Hondanoid' which is our collective nickname (can't remember who coined it--maybe FasDoc?) for these bikes which are based on old Honda designs, including the wheels. In other words, you can find sprockets and such easier because many aftermarket companies still make things for old Hondas. However, the 17"/17" setup uses a weird bolt pattern unrelated to old Hondas, for the rear sprockets, and I had to have one custom made for the '09, but had some bad luck with that sprocket (it was made for racing, not commuting, and seemed to eat these cheap chains, which stranded me a few times too many), so had to find an online bike accessories company out of the UK that had a steel aftermarket rear sprocket that fits my bolt pattern since the wheels are based on some European bikes as opposed to the common Hondas.

But, before you do anything to your sprocket, you being new to cycling, you probably should just keep the stock sprocket. It really depends on what kind of speeds you 'need' to maintain. If the roads you will be predominantly on are 45 mph roads, the stock one will be fine. If the speeds are 55 or faster, you will want to choose different roads! For reference, the final 8 miles of my 16 mile commute are on a 50 mph road. Lack of police has pushed the confidence of my fellow commuters up to 70+mph. I am the 'slow poke' doing around 60, which is about what the stock sprocket does. However, I have changed my rear sprocket from the stock 56 to a 48 (on bikes, to get higher top speeds, you change out the front sprocket to more teeth, and the rear sprocket to fewer teeth. You can do just one or the other, or both).

The TMEC comes with a 17 tooth front sprocket as stock, which is about as big as you are going to want or need for higher top speeds/less engine revs. The rear is a 56 tooth which limits your top speed to about 60max, but high revs and uncomfortable. I ordered a 48 tooth rear, which gives me a couple more mph top speed, but also makes cruising at 55 to top speed more palatable by bringing my revs down (keep in mind that everything vibrating tends to loosen bolts and things start falling off unexpectedly (back to locktite, LOL).

Ideally, I would like to find an aftermarket sprocket in the 44 tooth size.