How do you tell what size rear axle you have?

To know your rear axle size, start with the vehicle’s data tag or VIN-based build information, then confirm with any stamped codes or by checking key measurements if needed. Common rear-axle sizes you’ll encounter include Ford’s 8.8" and 9" designs, GM’s 7.5", 8.5" and 10.5" setups, and various Dana/Chrysler axles.

Where to start: rely on official data sources

Most reliable identification comes from manufacturer records, VIN decoding, and on-vehicle tags. Use these steps to pull the size from documentation before you begin any disassembly.

Before you begin the list, use the intro: The fastest way to identify size is to read the axle code or model printed on factory tags and match it to official references.



• Check the axle tag or sticker on the differential housing or cover for an axle code, ratio, or model designation (for example, a code that corresponds to 3.73, 4.10, or a model like “8.8” or “9”).


• Look up the VIN or build sheet (window sticker, original purchase documents, or a dealer’s database) to extract the axle specification that was installed at factory configuration.


• Consult the vehicle’s service manual or a factory parts catalog for decoding guides that translate codes into actual sizes and gear ratios.

Concluding note: If the tag or VIN data is readable, you can identify the size quickly and with high confidence. If not, move to physical identification or expert help.

Inspect the differential housing and stamped markings

Many rear axles carry explicit markings cast or stamped into the housing or on a removable tag. Reading these can reveal the model and gear size without removing major components.

Before you read the list: Look for clear markings near the differential cover bolts or on the side of the housing that indicate the axle size or ratio.



• Search for a stamped number like “8.8,” “9,” “10.5,” or other diameter indicators on the housing or cover. These markings often denote the ring gear diameter or axle model.


• Note any ratio codes (for example, “3.73” or “4.10”) and cross-check them with factory code charts for your make/model.


• Take clear photos of the markings from multiple angles to aid cross-referencing with catalogs or a parts specialist.

Concluding note: If you find a clear size or ratio marking, you can confirm the axle size by comparing it against manufacturer guides or a trusted parts database.

When tagging is unreadable: physical measurements and checks

If there’s no readable tag or stamping, you can corroborate the size by measuring or by comparing physical characteristics to known axle families. This approach is more involved and may require removing a cover.

Before you read the list: Measurements are a last resort when documentation is missing or illegible.



• Carefully remove the differential cover to expose the ring gear. Measure the ring gear diameter with calipers or a precise ruler; common diameters include roughly 7–8 inches (older GM/Chrysler), 8.8 inches (Ford), and 10.5 inches (larger GM/Chrysler trucks).


• Count the spline count on the axles or pinion shaft, if accessible, since different axles use different spline arrangements (for example, some common aftermarket IDs use 28- or 31-spline configurations).


• Measure the width between axle flanges or the overall axle housing width to distinguish between compact car axles and full-size or truck axles.

Concluding note: Measurements can confirm the size, but they’re best used to supplement tag/VIN information. When in doubt, consult a parts specialist with your measurements and vehicle details.

Common sizes and what they typically indicate

Here is a quick guide to sizes you’ll encounter across popular brands, to help you orient your search and avoid ordering the wrong part:

• Ford: 8.8" is the modern standard in many models; 9" is larger and older, often found in classic or high-performance setups; 7.5" or 7.625" are smaller, older configurations.


• GM (Chevrolet, GMC, etc.): 7.5" and 8.5" are common in older passenger cars and some light trucks; 10.5" is common in many trucks and some SUVs; 12-bolt or 14-bolt designs appear in larger/heavier applications.


• Chrysler/Dodge/Jeep: 9.25" and 8.25" are common in many modern cars and trucks; many heavy-duty models use Dana or other larger rear axles (e.g., Dana 44/60/70) depending on the vehicle.

Concluding note: Size guidance varies by model year and vehicle line. Use tags, VIN data, and, when needed, a professional readout to confirm exact size before sourcing parts.

What to do if you still can’t identify the size

When codes are unreadable or documentation is unavailable, you can:

Before you proceed: Consider a professional readout to avoid ordering the wrong part.



• Bring the vehicle information (VIN, approximate year, make, and model) to a trusted parts retailer or dealership and have them pull the axle specification from their database.


• Cross-check any measurements you’ve taken with catalogs that map dimensions to axle models, then order the correct size or have a technician verify in person.

Concluding note: In ambiguous cases, a hands-on check with a knowledgeable technician is the safest path to a correct identification.

Summary

To accurately determine your rear axle size, start with official data—VIN decode, factory tags, and manuals—and then verify with any stamped markings on the housing. If tags aren’t legible, you can measure the ring gear diameter after removing the cover and note the spline count or housing width as supplementary clues. Cross-reference all findings with manufacturer catalogs or a trusted parts supplier to ensure you have the correct size for maintenance, gear changes, or upgrades. This approach minimizes errors and helps you source compatible parts with confidence.

How do I identify my rear axle?

To identify your rear axle, first look for an ID tag or stamped numbers on the axle housing, which may indicate the model, gear ratio, and other specs. If tags are missing, use visual identification by counting the bolts on the differential cover, noting the cover's shape, and measuring the ring gear diameter and bolt pattern. You can also check for stamped numbers on the axle tubes or on the differential cover itself, often near the fill plug. 
Using ID tags and stamps 

• Find the tag: Look for a metal tag bolted to the differential cover or a stamp on the axle tube. Trailer axles often have a sticker on the back or side. 
• Decode the information: The tag or stamp may include codes for the gear ratio, whether it has a limited-slip differential, and the axle's manufacturing plant and date. 
• For Ford axles: Look for the tag on the differential cover. Numbers on this tag often contain information about the gear ratio and type, with specific codes for different model years, says HOT ROD Network. 
• For trailer axles: Labels are often on the back of the axle and include the serial number, capacity, and build details, according to Dexter Group. 
• For GM axles: Numbers may be stamped on the axle tubes and you can decode them using a chart. The ratio may also be stamped on the ring gear after removing the cover, with the format [pinion teeth]-[ring gear teeth], notes Speedway Motors. 

This video explains how to read a Dexter axle label: 47sDEXTER TVYouTube · Aug 22, 2022
Using visual identification
You can watch this video to see a visual comparison of different rear axles: 28sJeep Trails & Barbells YouTube · Nov 23, 2022

• Differential cover bolts: Count the number of bolts that hold the differential cover on. Different axle models have different bolt patterns, according to Sierra Gear & Axle. 
• Cover shape: Note the shape of the differential cover, as it can be a key identifier. For example, some Dana axles have covers that look identical to their front-wheel-drive counterparts, explains this YouTube video. 
• Ring gear diameter: Measure the diameter of the ring gear. You may need to remove the cover to do this, says Sierra Gear & Axle. 
• Axle tube shape: Check if the axle tubes are a consistent diameter all the way out or if they taper towards the ends. For example, a 7.5-inch axle has a consistent diameter, while an 8.5-inch has tapered tubes, as shown in this Facebook post. 

How do I know what size axle I have?

To measure an axle, use a tape measure to record its specific dimensions, which vary by type but typically include hub face-to-hub face (overall width for trailers), spring center-to-spring center (trailer spring placement), or length from a mounting point to the end of the shaft (for axle shafts). Bicycle thru-axles are measured by overall length and diameter, with the length being from the head to the tip of the threads. 
For a trailer axle

• Hub Face: The most common measurement for overall width. Place a tape measure on the outside of one hub (where the wheel rests) and measure across to the outside of the other hub. 
• Spring Center: Measure from the center of the leaf spring seat on one side of the axle to the center of the seat on the other side. 
• Diameter: Measure the main body of the axle tube. This can be done with a tape measure or caliper. This is used to determine the axle's weight capacity, as different diameters correspond to different capacities. 
• Brake Flange (if applicable): Measure from the outside face of one brake flange to the outside face of the other. 

This video demonstrates how to measure trailer axle hub face and spring center: 1metrailerYouTube · Jun 12, 2023
For an axle shaft (vehicle)

• Semi-Float/C-Clip Axles: Measure from the end of the shaft to the outside mounting flange. 
• Full-Float Axles: Measure from the front surface of the mounting flange to the end of the spline. 
• Front Axle Shafts (with U-joint): Measure from the center of the U-joint to the end of the shaft. 

For a bicycle thru-axle

• Overall Length: Measure from the base of the head to the tip of the threads. 
• Diameter: Measure the diameter of the axle shaft itself, often with calipers. 
• O.L.D. (Over Locknut Dimension): For the thru-axle, this is the distance from the inside of the fork or frame dropout to the inside of the other dropout. 

This video explains how to measure bicycle axle O.L.D. and length: 56sThe TrailerSmithYouTube · Jun 30, 2022

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 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.