different track sizes, what model it is (X or not) will have different gears in them
sometimes, it will be different right from the dealer depending where it was purchased
easiest way to tell what you have....pull the chaincase cover off
i have an 06 X 151 that was purchased in Alberta - 21/45 gearing
i changed it out to a 20/45 but went w a 15w set up
20th 15w top gear w 440 spacer
45th 13w btm gear
all wrapped up in a 74 link 15w chain
the 06's were known to blow chains
it is cheaper to replace the top gear, but any more than a 1 tooth drop and i would be changing out the bottom gear instead
19/45 puts too much of a strain on your chain and creates steep angles or turns
i personally like keeping the ratio around 2.24, hense why the 20/45 set up