19/40 is geared down from stock, which is 20/39. What are you looking to gear(which direction)?? I believe if you go more than 19/40 you have to change your chain... i THINK. not sure. 20/39 will get ya 78.8mph @8000 rpm, 88.6 @ 9000 rpm. 19/40 will get ya 72mph @ 8000, and 81.2 @ 9000. All these specs are right out of the Cat factory manual.