The drive axle is said to have a certain axle ratio (meaning the amount which the rear axle reduces the turning speed of the engine/transmission as it transmits this motion to the rear wheels. This number is actually a comparison of the number of gear teeth on the ring gear and pinion gear. For example, a 4.11 rear means that there are 4.11 teeth on the ring gear for every tooth on the pinion gear. Put another way, the driveshaft must turn 4.11 times to turn the rear wheels once. Actually, on a 4.11 rear, there might be 37 teeth on the ring gear and 9 on the pinion gear. By dividing the number of teeth on the pinion gear into the number of teeth on the ring gear, the numerical axle ratio is obtained. This also provides a good method of ascertaining the axle ratio with which your car is equipped.
Another, less accurate but quicker, method of determining gear ratio is to jack up and support the car so that BOTH rear wheels are off the ground. Make a chalk mark on the rear wheel and driveshaft, then place the transmission in Neutral and turn the rear wheel one complete turn (exactly). While turning the rear wheel, count the number of turns that the driveshaft makes (an assistant makes this a little easier). The number of turns made by the driveshaft (during one complete rotation of the rear wheel) is an approximation of the rear axle ratio. Again, if the driveshaft turned just a little bit more than 4 times, you have a 4.11 or similar gear ratio.