Please Note: Does Require Pressing The Gears Off the Shaft 2.88:1 Final Drive Gear Ratio (usually 3.41-3.71 stock) Average Top Speed Gain on Flat Ground is 7 to 10 mph (Depends on other Modifications on your Bike) These Are Great Addition for The 139QMB 4 Stroke 50cc Chinese Scooter and ATV Engines with Standard or Big Bore Kit. Product Description 1.New Products with High Quality 2.If your Engine is Revving to High and Bike is Going to Slow Then This Gear Set Will Help 3.Fits Most of Chinese Made Scooters and ATV. Such as JONWAY, JMSTAR, ROKETA, SUNL, TANK, PEACE, TAOTAO, Dongfang, Kazuma, JCL, BMS