well , its very easy , there are different gear ratio's , longer , bit longer etc.
to know what your gearing is now you'll have to know what is std.
here is a link to some gear ratio's , it's in Dutch but i think you'll understand
http://forum.scooterforum.net/forum/showthread.php?t=83873
there is a Primary gear set and a secondary gear set , so you can make a lot of different gear ratio's..
depending on what you're standard gear ratio is you can change your gear ratio , you'll have to know what you want , you can't just use any gear ratio on any engine and how the bike is used is also important..
the Malossi Springdrive is a gear set with is espacialy made for off-road use.
it's very expensive btw