The 20 tooth sprocket has 3 more teeth than the stock sprocket on the dirt dog and x-racer models. This will give you about 3mph more top speed.

ScooterX - 2010+ 49cc Dirt Dog, 2009-2010+ 49cc X-Racer and ESX Racer

Will also fit many other chinese gas scooters, go karts, mini choppers, and pocket bikes on the market.

If you are not buying this for a ScooterX product just make sure you're scooter uses the same style transmission or gear box as ScooterX. It is the most common Transmission on the market for this type of product.