How do you tell what size differential you have?

The size of a differential is typically described by its gear ratio—the number of turns the driveshaft must make for one full wheel rotation (for example, 3.73:1 or 4.10:1). In some cases, “size” also refers to the physical ring-gear diameter (such as 7.5", 8.8"). You can determine this by checking labels, your VIN/build sheet, or by counting gear teeth or observing driveshaft-to-wheel rotations.

In this guide, we explain what “size” means, how to locate it on your vehicle, and practical steps to identify your differential size without guessing. We cover quick label checks, hands-on tooth counting, and a safe driveshaft rotation test, plus notes on identifying the differential type (open, limited-slip, or locking) which can affect sizing decisions.

Fast checks you can do without taking things apart

These steps rely on factory labeling and documented data to confirm the axle size quickly.

• Look for an axle or differential tag stamped on the differential housing or cover. Common markings include gear ratios like 3.55, 3.73, or 4.10.


• Check the vehicle’s door jamb sticker or the glovebox/build sheet for an entry labeled “AXLE” or “DIFF” showing the gear ratio.


• Use the VIN to pull the build sheet or an online decoder to reveal the axle ratio for your exact model and year.


• Consult the owner's manual or factory service manual for your specific model/year; the ratio is often listed there.

If you find a code, you can cross-reference it with manufacturer data to confirm both the ratio and the axle family.

Counting teeth: a hands-on method to verify the ratio

When labeling isn’t accessible or you want to verify, counting teeth on the gears inside the differential provides a direct determination of the ratio.

1. Safely access the differential (you may need to remove the differential cover and support the vehicle as needed).


2. Count the teeth on the ring gear (the larger gear attached to the case).


3. Count the teeth on the pinion gear (the smaller gear that meshes with the ring gear).


4. Compute the ratio: gear ratio = ring gear teeth ÷ pinion teeth. For example, 41 ring teeth and 14 pinion teeth yield 2.93:1; typical passenger-car ranges run roughly from 2.73:1 to 4.56:1 or thereabouts.

Note: If you open the differential, replace the gasket and refill with the correct oil to avoid leaks and contamination.

Alternate method: driveshaft-to-wheel rotation test

Another non-destructive way to estimate the ratio is to observe how many times the wheel turns for one full turn of the driveshaft. Do this only when the vehicle is safely supported and in appropriate drivetrain state (neutral or with the wheels off the ground as applicable).

1. Secure the vehicle and mark a reference point on the driveshaft and a wheel.


2. Rotate the driveshaft exactly once and count how many wheel revolutions occur.


3. The gear ratio is approximately the number of wheel revolutions per driveshaft revolution (e.g., 4 wheel revolutions per 1 driveshaft turn ≈ 4.00:1).

This method offers a quick cross-check but can be influenced by transmission state and drivetrain design, so use it as a sanity check rather than the sole determinant.

Ring-gear diameter vs. ratio: understanding the “size” beyond gears

In some contexts, “size” refers to the physical ring-gear diameter or the overall axle housing size (for example, 7.5", 8.8" ring gears). The diameter is tied to the axle family and is not a substitute for the gear ratio. To identify diameter, you may:

• Identify the axle family from stamping on the housing or from the axle code, then consult the manufacturer’s spec sheet.


• Measure the ring-gear outer diameter with appropriate tools if you have access to the gears, and compare to known sizes for your axle family.


• Cross-check the vehicle’s service manual or parts catalog for the correct ring-gear size for your exact axle model.

Knowing both the ratio and the ring-gear size helps ensure compatible parts, accurate speedometer calibration, and appropriate performance upgrades.

Notes on identifying the differential type

Beyond size, knowing whether you have an open, limited-slip (LSD), or locking differential is important for maintenance and upgrades. Some units are clearly labeled; others require inspecting the internals or consulting the build data. If in doubt, reference the build sheet or contact a dealership or parts professional with your VIN.

Summary

To determine your differential size, start with easy checks: axle tags, VIN/build sheet data, or the door sticker. If those sources aren’t available, count the teeth on the ring and pinion or use a driveshaft-to-wheel rotation test to estimate the gear ratio. Remember to verify whether the differential is open, LSD, or locking, as this affects interpretation and compatibility. With the gear ratio and, if needed, ring-gear size identified, you can plan maintenance, replacements, and upgrades with confidence.

How do you tell if you have a 7.5 or 8.5 rear end?

You can tell the difference between a 7.5 and 8.5 rear end by looking at the shape of the cover, the shape of the axle tubes, and the protrusions on the bottom of the housing. The 8.5 has a round cover with larger, rectangular protrusions at the bottom, while the 7.5 has a more squared or oval cover with smaller, crescent-shaped lugs at the bottom. Additionally, the axle tubes on an 8.5 are flared or stepped down as they enter the pumpkin, while the 7.5 has straight axle tubes.
 
This video shows how to visually identify a GM 10 bolt rear axle, including how to distinguish between a 7.5 and 8.5 rear end: 53sandhotrodshopYouTube · Dec 14, 2023
Visual differences

• Cover shape: An 8.5 has a round rear cover, whereas the 7.5 has a more squared-off or oval cover. 
• Axle tubes: The tubes on an 8.5 will appear to swell or flare out just before entering the differential housing. The 7.5 has straight axle tubes. 
• Bottom protrusions: The 8.5 has larger, rectangular "ears" or protrusions at the bottom of the housing (around the 5 and 7 o'clock positions). The 7.5's protrusions are smaller and more crescent-shaped. 

Other methods

• Measure the bolts: On some GM 10-bolt rears, the distance between the bottom center bolt and the nearest bolt on either side can be used for identification. 
  • 8.5: The distance is approximately 3.753.753.75 inches. 
  • 7.5: The distance is approximately 3.253.253.25 inches. 
• Check the vehicle's RPO codes: Look for a sticker in the driver's side doorjamb or glove box. The codes listed under "Axle" or "RPO" will specify the factory-installed rear end. This is the most accurate method if the rear end is original. 

How to tell if you have a dana 30 or 44?

You can tell if you have a Dana 30 or 44 by checking the differential cover shape, which is different for each model, or by looking for a cast-in model number on the axle itself. The Dana 44 has a larger ring gear and a more distinctively shaped differential cover compared to the Dana 30. 
Differential cover shape

• Dana 44: Has a more unique, often odd-shaped differential cover.
• Dana 30: Features a more "concentric" or rounded shape. 

Model number

• Look for a number like "44" or "30" cast directly into the metal, often on a reinforcing rib on the lower right side of the differential housing. 
• A smaller tag might be bolted to the differential cover, especially on later models like the 1993-1998 Grand Cherokee (ZJ). 

Two-piece vs. one-piece axle shafts (rear axles) 

• Dana 30: All Dana 30s are two-piece axles, meaning they have a large nut on the end of the axle shaft. 
• Dana 44: Can be either two-piece or one-piece. A one-piece Dana 44 will have a solid flange on the end instead of a nut. 

How do I know what size my differential is?

To measure the "differential size," you can measure the width of the rear axle housing or the diameter of the ring gear. Width is measured from wheel mounting surface to wheel mounting surface, while the ring gear diameter is measured across its widest point, often after removing the differential cover. Other methods include counting cover bolts or checking for a pinion support, which are also ways to identify the differential. 
This video explains how to measure the width of a rear axle housing: 53sCurrie EnterprisesYouTube · Nov 29, 2023
Measuring axle housing width 

• Method 1: Wheel-to-wheel (WMS) width 
  1. Use a straight edge and secure it to the wheel hub mounting surface or brake rotor/drum on each side. 
  2. Measure the distance between the inside of the two straight edges with a tape measure. 
  3. This gives you the "wheel mount to wheel mount" (WMS) surface width. 
• Method 2: Housing and axle flange-to-flange 
  1. Measure the distance from one housing flange face to the other. 
  2. Measure the distance from one axle flange face to the other. 
  3. Note the brake offset, which is the distance from the housing flange face to the axle flange face, as this can add to the overall width. 

Measuring ring gear size

• Method 1: Ring gear diameter
  1. This measurement requires removing the differential cover. 
  2. Place a tape measure across the ring gear's widest point, from tip to tip of the teeth. 
  3. This is a good way to identify the differential model and gear ratio. 
• Method 2: Ring gear stamping
  1. With the cover removed, find the numbers stamped on the ring gear. 
  2. A common stamping format is XXYY, where XX is the number of teeth on the ring gear, and YY is the number of teeth on the pinion gear. 
  3. Divide the ring gear teeth by the pinion gear teeth to get the gear ratio. 

Other identification methods

• Count cover bolts: Some differentials can be identified by the number of bolts holding the cover on. 
• Check for a pinion support: Some differentials, like the Ford 9-inch, have a pinion support that bolts to the front of the housing, making it easier to identify. 
• Look for a tag: Check for a Bill of Materials (BOM) tag on the axle tube, which can provide model, gear ratio, and other component information. 

How do I know if I have a 3.73 or 4.10 gear?

And since we live in the day and age of 2025. Uh if you're curious. You can easily. Google you know uh the number of teeth on a ring gear.