i run 14/39
its the sprocket sizes front and back, you can change gearing depending on if you want more topend or more torque.
14t is 1 size bigger then stock at 13t and a stock rear is 40 so this would result in a slightly higher top speed, now if you go down on the back sprocket that actully brings up your top speed aswell. its the opposite of the front.
so:
bigger back-more torque
bigger front-more top speed
smaller rear-more top speed
smaller front-more torque
& smaller rear tires = more torque
bigger rear tires= more top speed
hope i explained it right haha.