Use a measuring tape to calculate the outer surface area of your tire. The simplest way to do this is to place the tape on the ground and ask a helper to drive the car slowly in straight direction, while you wrap it around the tire as the wheel rotates.
Put a jack at the rear side of the vehicle and keep pumping the air in the cylinder till the tires are about 4-5 inches clear of the ground. As you will have to lie underneath the car for the next part, it is recommended to place two fail safe load bearing stands on either sides of the car to ensure maximum safety.
Release the pin in the hydraulic jack so that the car rests only on those two jacks. Spread an old cloth under the surface of the car and don your working clothes. Use flashlight to identify the rear axle. Put one feet long masking tape horizontally on the drive shaft so that it is easily visible from the side of the car, near the rear wheels.
Put a piece of making tape on one of the tires as well. Remove the hand brakes and give that tire one complete rotation. Measure the distance travelled by the masking tape on the shaft. If the distance is too small to be measured, rotate the tire about 10 times to get a better reading.
Measure the number of revolutions the drive shaft completes during the 10 rotations of the vehicle. The formula for calculating the gear ratio is quite simple. The gearing ratio of an axle is equal to the number of rotations taken by the drive shaft ratio the number of rotations of the wheel.
For example if the drive shaft turned 3.25 times in one rotation of tire, your gear ration is 3.25: 1.