Jack the rear end up on one side only, leave the other tire touching the floor so it won't turn. Mark the driveshaft and see where the valve stem is pointing (like 1:00 oclock, 8:00 oclock etc.) If your by yourself mark the inside of the tire so you can turn it TWO revolutions while watching the driveshaft. Count the number of times the driveshaft turns in two tire revolutions and that is the gear ratio. A 4.11 will be four turns plus a smidgen, a 3.90 will be not quite four turns, a 3.55 will be three and a half turns, etc.
