Figure out gear ratio?

My Buddy has two suregrip chunks.The first is a 489 case 323 SG.The second is a 741 case but I do not see a gear ratio stamped into the ring gear.Matter of fact the ring gear on the 741 case is much beefer than the one in the 489 case.The teeth are bigger and the ring gear is wider.I did see a 6 or 7 digit number a part number maybe.Also a rectangle logo maybe.So I am thinking a 4:10-5:30 aftermarket ring and pinion in the 741.I know there is some sort of formula tofigure out the gear ratio.Can some one give that formula? Thank You,Jim