What is the best sprocket to suzuki thunder 125 gain more speed?

The best sprocket to fit a Suzuki Thunder 125 motorcycle to gain more speed is a 14-tooth front sprocket paired with a 42-tooth rear sprocket. This combination will give the bike a higher top speed, but will also reduce its acceleration slightly. If you are looking for better acceleration, you can stick to the stock 15-tooth front sprocket and install a 44-tooth rear sprocket. This will give you slightly slower top speeds but better low-end power. Keep in mind that the actual performance improvement may vary depending on your riding conditions, motorcycle specifications, weight of the rider, etc.