Look for a metal tag held on by two of the differential cover bolts. This tag will have the gear ratio stamped into it. This may not be accurate though for used vehicles or vehicles with a questionable past.
Another way to determine the ratio is from the bill of materials stamped into the housing. This is only good for Dana brand axles and the number can be decyphered from their website.
One way to be positive of the gear ratio is to remove the cover and count the teeth on the ring gear, and divide by the teeth on the pinion gear to get the ratio.
Example: 39 Ring Gear teeth / 11 Pinion Teeth = 3.545 or a 3.55 Differential Ratio
A quick way to get a general idea about the ratio try the following:
Jack up the rear axle and support it by jack stands.
Put a chalk mark on the top of the tire and the top of the driveshaft.
Put the transmission in neutral and spin the rear wheels.
Count the number of times the drive shaft spins to one revolution of the tire.
Example: If the drive shaft spins 3.5 times to one revolution to the tire is roughly 3.55; 4 times is 4.10; 3 and 3/4 times is roughly 3.73 and so on.
As the pinion gets smaller with a lower (numerically higher) gear ratio the ring gear has to be moved over to be in contact with the pinion. One way to facilitate this is to make the ring gear thicker but this can only be done to a certain thickness. Once the max thickness of the ring gear is reached the gear carrier must be changed. These are called Carrier Breaks and they are gear ratio specific. What makes them so specific is that the mounting flange for the ring gear is moved over to allow for proper gear engagement between the pinion gear and the ring gear.
Gear carrier breaks (numerically):
Dana 30 3.55 and down, 3.73 and up
Dana 35 3.08 and down, 3.55 and up
Dana 44 4.88 and down, 4.89 and up
Example: You have a Dana 30 with 3.08 ratio, but you wish to go with 4.56 ratio. You will have to purchase a new carrier since the break is at 3.55 and 4.56 is numerically higher than 3.55.
Every attempt has been made to make the above information as accurate as possible. However, with various options or packages the gear ratios may differ than the information above. Please verify your ratio using the above tips to be absolute on what ratio you have.
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