Re: Ford 9\"
Since you are most likely going to take the rear end apart you might try counting the number of teeth on the ring gear and dividing that by the number of teeth on the pinion gear. If you have 41 teeth on the ring gear and 10 on the pinion gear you get a 4.10 gear ratio. Another option is to set the rear end up on blocks or stands of some type... you might need two people or a mirror to do this... Make a mark on one of the outer hubs and note its location, 1-o'clock, 5-o'clock etc... and another on the yoke at the 3rd member... Then you turn the yoke (pinion) very slowly and count how many times the hub goes around. If you turn the yoke one complete revolution and the hub spins just a nats ass over 4 times, you have a 4.10... if you turn the yoke one revolution and the hub turns about 3-3/4 turns, you probably have a 3.73. If you make accurate marks and pay close attention, you can pretty accurately calculate what ratio you've got.
Decoding the numbers isn't always accurate - I decoded a junkyard 9" a couple years ago and bummed out when I got it home, opened it up, and realized someone had performed a nasty gear swap with some cheapo 3.00-3.08 gears.