The casting number on the housing tells you exactly nothing about the gear ratio. Either pull the cover plate and post the number stamped on the perimeter of the ring gear (41-10 for instance, indicating a 4.10:1 ratio) or turn one wheel with the other one on the ground and count the turns of the driveshaft. Be aware that the spider gears will be involved, so turn the wheel TWO revolutions while counting the driveshaft rotations. Pulling the cover is the most accurate way, and that also allows you to inspect the gears for wear and/or damage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!