Sprockets are an easy mod that will change how quickly your bike will get into (and how long it will stay at) a given RPM. Put more simply, they will allow you to accelerate faster or reach a higher top speed (but usually not both). You basically get to choose one of the following:
1. A slightly higher top speed with slower acceleration to reach it.
2. Faster acceleration but a slightly lower top speed.
Which would you pick? Keep in mind, your engine isn’t getting more powerful, it is just using the available power in a different way.
The Grom comes stock with a front sprocket that has 15 teeth and a rear sprocket that has 34 teeth. By dividing the two numbers you get a ratio. (34 divided by 15 = 2.26666… or simply 2.27.) As the gear ratio changes, the bike will perform differently.
There are a wide range of sprockets to choose from so you can mix and match until you find something you are comfortable with. By doing some simple math, you can get a pretty good idea what that gear ratio is going to feel like. And, just in case you didn’t feel like doing the math yourself, here’s a chart that does it for you. The numbers in bold are stock sprockets (meaning you would have less to buy if you want to run that gear ratio).
|16||34||2.125||Probably great if you have a high-performance monster.|
|17||37||2.18||Note: I don’t know if anyone actually sells a size 17 front sprocket.|
|15||35||2.33||Slight acceleration gain, no speed loss|
|15||36||2.40||Moderate acceleration gain, slight speed loss|
|14||34||2.43||Very popular way to improve acceleration for less than $10|
|15||37||2.47||Substantial acceleration gain and speed loss|
|13||34||2.62||Kart track use only IMHO|
Chart content stolen from MNNTHBX. http://www.hondagrom.net/forums/7-grom-talk/3637-going-2-up-rear-sprocket-3.html#post106299
Keep in mind that dropping 1 tooth in the front is nearly equal to adding 2 teeth in the back. For example, changing the stock 15T front to a 14T front = 2.43. That's nearly the same as keeping the 15T in front but changing the rear from a stock 34T to a 36T which equals 2.40. Many people prefer changing the front sprocket rather than the rear because it only costs about $8 for a front sprocket but can cost upwards of $30 for a rear sprocket.
The general consensus is that the stock chain is crap and it wears out quickly anyway. Can you still run a stock chain with different gears? Yes, to a point. It's possible to run a 14/36 combo on the stock chain but if you want to go bigger than that for a stunt set up, you're probably going to need more chain.
I'm not going to get into the benefits of different chain types here because this is a post about sprockets. If you want to read highly opinionated thoughts about chains, you can find them on any motorcycle forum in the world.