This might help:
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Possibly the easiest mod that you can do which will have a pretty drastic change to the way your bike performs is to change the final drive gearing.
The final drive of the GZ250 is a simple chain/sprocket combo. The final drive ratio is
2.73. This number is easily calculated by dividing the number teeth on the rear sprocket by the number of teeth on the front sprocket. In our case, since we have a factory sprocket combination of 15 and 41 (15 teeth in the front and 41 in the rear) the calculation would look like this.
41 / 15 = 2.73333333 (or 2.73)
The lower the gearing, the lower the revs. The higher the gearing, the higher the revs.
You'll almost never want to change any sprocket more than one or two teeth sizes in any direction over the factory number.
By comparison, switching over to a 16T front sprocket lowers the final drive ratio to 2.56 (41/16)
What does this mean in practical application?
The 16T front sprocket lowers cruising revs in all gears and allows the lower gears to be used for a longer period of time. However, you will probably notice a drop off in your top speed. While the factory gearing allows the GZ to reach somewhere around 80mph, the 16T front sprocket will lower that to around 70mph. The reason being that the internal gearing of 5th gear has a ratio of .818, which is basically an internal overdrive and seems to make the gear too "tall" for the mighty GZ to handle.
Where can I purchase a new sprocket?
The Suzuki factory part number for a 16T sprocket is
27511-37200
The JT Sprockets part number is
JTF 434.16
Sprocket Specialists make both front and rear sprockets for the GZ250.
The front sprockets are available from size 10-16. Part number:
528-(number of teeth)
The rear sprockets are available from size 28-74. Part number:
461-(number of teeth)
How do I change my front sprocket?
http://www.gz250bike.com/viewtopic.php?t=317