Between your car's engine and its wheels lay a number of gears and gearsets used to multiply torque and optimize engine rpm relative to road speed. The transmission does most of the work, constantly shifting ratios according to road speed, but the axle "final drive" ratio provides a constant differential between engine rpm and wheel speed. The terminology is a bit confusing, since numerically lower ratios are referred to as "higher" gearing, while numerically higher gearing is "lower." Higher gearing reduces engine rpm for fuel economy and top speed, while lower gearing favors acceleration.
Things You'll Need
 Measuring tape
 Calculator
 Reference material

Measure the outside circumference of your tire with the measuring tape. The simplest way to do this is to unroll your measuring tape in front of the wheel and drive the car forward onto it.

Park the car, pick up the ends of the tape measure and wrap them around the outside of your wheel. Note the distance in inches. For our example, we'll use a 5.0liter Mustang with large drag slicks; its rear tires measure 103 inches in circumference.

Divide 63,360 (the number of inches in a mile) by your tires circumference to determine how many times that your tires will turn in a mile. The drag slicks on our example Mustang will turn 615 times in a mile.

Determine your transmission's directdrive gear, or the gear in which it is turning at a 1to1 ratio. Direct drive is usually third gear on a threespeed, fourth gear on a four, five or sixspeed and fifth gear on a sevenspeed or higher.

Find a long, straight stretch of road, place your gear selector into the directdrive gear and set your cruise control at exactly 60 mph (one mile per minute). Note the engine rpm on your tachometer. Our example car is a fivespeed, and that the tachometer reads 2,500 rpm at 60 mph.

Divide your observed engine rpm at 60 mph by the number of times that the tire revolves per mile to derive the rear axle ratio. In our case, we'll divide 2,500 by 615, which comes out to 4.06; thus, this car's rear axle ratio is 4.06to1.
Tips & Warnings
 Alternatively, you could jack the car up, spin the one drive tire and count the number of times that the tire turns per driveshaft revolution. However, this will only work with rearwheeldrive vehicles, as frontwheeldrive cars don't have exposed driveshafts.
 Note that some automatic transmission cars and a few manuals don't actually have a directdrive gear. However, they'll almost always have one that's within about 0.10 of a true 1.00 to 1.00. If your nearest gear is over 1.00 (say 1.07 to one), then subtract 1.00, multiply that by the gear ratio that you derived above and add the end result to the gear ratio. For example, 1.07 minus 1.00 equals 0.07. That 0.07 multiplied by 4.06 equals 0.28, which added to 4.06 gives us a 4.34to1 ratio. If your closest drive ratio is below 1.00 (say 0.97), then subtract it from 1.00, multiply that by the observed ratio and then subtract that figure from the observed ratio. For example: subtract 0.97 from 10.00 (equals 0.03), multiply that by 4.06 (equals 0.12) and then subtract that from 4.06 to derive a final drive ratio of 3.94 to one. It's not a mathematically perfect method, but it'll get you very close.
References
 "Race Car Engineering and Mechanics"; Paul Van Valkenburgh; 2001
 "How to Drag Race"; Kevin McKenna; 2008