How Do I Determine Gear Ratio on Ford Rear End?

by Tim Brugger
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There are a couple of ways to determine the gear ratio for a Ford rear end, or any car or truck for that matter, and we will discuss both. First, it may be helpful to review what the gear ratio is and the impact of it on the performance of your car or truck. The gear ratio, along with overall speed, revolutions per minute (RPM) and tire diameter all factor into an automobile's acceleration capacity. The higher the ratio (the revolutions), the better the acceleration. However, the more acceleration the more wear and tear on the engine, and the worse your gas mileage.

Step 1

Getting an estimate can give you a pretty good sense of your Ford's gear ratio, and is fairly easy to do. First, put the rear of the car or truck on jacks. Then wrap a cable or strong rope around the driveshaft, and hold the cable/rope straight up in the air (in a 12:00 position), making certain there is plenty of length remaining, as it will wind around the driveshaft.

Step 2

Mark the tire, clearly, with white chalk or an easy to see marker that is not permanent. Place the mark directly at the bottom of the tire. After making certain the Ford car or truck is in neutral, rotate the tire exactly one time, until the chalk mark is back in its original position, while counting the number of times the driveshaft rotates. Monitoring the cable or rope revolutions will assist in counting. Try and be as precise as possible when counting the driveshaft rotations.

Step 3

Measuring the gear ratio using this estimating technique is as follows: assume the driveshaft rotated 4 times, and the cable or rope is left, after the 4 rotations, at the 1:00 position. This would equate to the Ford's gear ratio being approximately 4:1; four complete rotations of the driveshaft, and then left at the 1:00 position after rotating the tire one complete revolution. Again, not an exact science, but this will provide you with a good estimate.

Step 4

Measuring the precise gear ratio involves the counting, and subsequent dividing, of the number of ring gear teeth by the number of pinion gear teeth. For example, if your Ford car or truck has a ring gear with 38 teeth, and the pinion gear has 10 teeth, the gear ratio is 3.8:1 (38/10).

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