take the cover off, and with the ring gear to your right, adjacent to the mounting ring, wipe the sludge? off of the 3/8"ish edge while rotating the gears, and you'll find numbers like 3797341-10-39. and dividing 39 by 10 you get 3.9, or 3.90s. like joe h said, just divide the larger number by the smaller number. it also gives you an opportunity to eyeball the teeth's wear pattern etc
