Determining Gear Ratio

The absolute way to establish gear ratio is to:

1.Count the teeth on the ring and pinion

2. Then divide the tooth count of the ring gear by the tooth count of the pinion gear (carry out to 2 decimal points).

Example: 41 tooth ring gear divided by a 10 tooth pinion gear yields a 4:10 to 1 gear ratio.

Determining the ratio from the tags:

If your axle has not been worked on and the tag has not been lost they will give you a ratio. They are usually attached under one of the cover bolts. It may give you a direct reading or the tooth count. Dana axle will also give you a "Bill of Material" number. With this number we can tell you about any thing you want to know about the axle. Later Ford axles will also give you a code number which determines ratio and ring gear size.

Estimating the axle ratio from tire rotation:

A reasonably reliable way to determine axle ratio is to count the wheel and drive shaft rotations. Select a level surface to conduct the test. Mark the edge of the tire (bottom) and a spot on the drive shaft so you can count the rotations. Roll the vehicle so the tire makes one complete rotation. Count the number of times the drive shaft rotates. If the shaft turns three and a half times you will have about a 3.50 ratio. If it turns three and three quarters times then you have a 3.73 ratio. If it turns five and one third times then you have 5.38 ratio. And so on.

Most ring gears have numbers stamped on them. So when they are exposed you will also be able to identify the ratios.