How do I know what size my differential is?
Bottom line: determining your differential size means identifying its gear ratio and ring gear diameter. You can find this information on a factory tag, in the vehicle's build sheet or service manual, or by measuring the gears yourself. Here's how to verify it and why it matters.
Start with the factory tag or axle code
Many differentials carry a stamped or engraved code that encodes the gear ratio and sometimes the ring gear size. Reading this tag is the fastest route to an answer.
- Locate the tag: look on the differential cover, the housing, or a small plate fastened to the axle.
- Read the code or numbers: you may see a direct ratio like 3.55 or a code that requires a cross-reference.
- Cross-reference with official docs: use your vehicle manual, service guide, or a manufacturer axle-code chart to translate the tag into a ratio and size.
- Be mindful of variations: different trims or regions can use different axles; verify for your exact vehicle.
The factory tag is usually the quickest and most reliable source for determining your differential size.
Count teeth to calculate the gear ratio
If the tag is unreadable or missing, you can determine the size by counting gear teeth and performing a simple calculation.
- Access the gears safely: remove the differential cover and inspect the ring gear and pinion while following proper safety procedures.
- Count the ring gear teeth: count how many teeth are on the ring gear, or count the teeth that mesh when the ring gear is rotated through one full turn.
- Count the pinion teeth: count the teeth on the pinion gear that engage the ring gear.
- Compute the ratio: gear ratio = ring gear teeth / pinion teeth (example: 41 ring teeth and 14 pinion teeth = 2.93:1).
- Verify and recheck: recount to confirm accuracy and consider a second person to help count.
Measuring teeth directly provides a precise ratio when tags are unavailable, but it requires care and proper tools.
Use VIN/build sheet or dealer resources
Vehicle history and current documentation can reveal axle specification, especially on newer models where the factory records are digitized.
- Find the axle code on the VIN or build sheet: look for an axle/gear ratio entry on the door jamb sticker, under the rear seat, or in the vehicle’s build sheet.
- Decode the code: use an official VIN decoder or a dealer resource to translate the code into the numeric ratio and ring gear size.
- Validate with a dealer if needed: when in doubt, a dealership can pull factory configuration data for your VIN.
VIN or build-sheet verification is a reliable secondary method, especially for confirming rare or optional axle configurations.
What "size" means and how it affects parts
In axle terminology, "size" typically refers to two related attributes: the gear ratio (how many turns of the driveshaft per wheel revolution) and the ring-gear diameter or model class (which affects strength and parts compatibility). Knowing both ensures you order the right gears, bearings, and seals and avoids mismatches when upgrading or replacing components.
Practical considerations for common setups
Many light-duty axles use gear ratios commonly found in passenger cars and light trucks, and ring-gear diameters in the 7.5" to 8.8" class, depending on the model and market. Exact figures vary by vehicle, year, and configuration. Always confirm against your specific VIN or tag before ordering parts.
Summary
Determining the size of your differential involves identifying its gear ratio and ring gear diameter. Start with the factory tag or axle code for a quick read, verify with a teeth-count if needed, and cross-check with VIN/build-sheet data. Knowing both numbers helps ensure proper parts compatibility and informed gear-up decisions.
Does the VIN number tell you the gear ratio?
You may be able to tell the gear ratio from a vehicle's VIN by using an online VIN decoder, though not all manufacturers include this information in the VIN itself. If the gear ratio is included, it will likely be in the Vehicle Descriptor Section (VDS), which are the 4th through 8th digits of the VIN. For many vehicles, a sticker on the driver's side door jamb or on the vehicle's build sheet or owner's manual is a more reliable way to find the gear ratio.
How to check the VIN
- Use an online VIN decoder: Enter the full VIN into a free online decoder. Some manufacturers have their own decoders, such as Mopar's decoder.
- Look for the gear ratio code: Some VINs will have a code within the VDS that represents the gear ratio (e.g., "3731").
- Consult the manufacturer: If the VIN decoder doesn't provide the information, contact the vehicle manufacturer for assistance.
Other methods to find the gear ratio
- Check the vehicle sticker: Look for a sticker on the driver's side door jamb or on the vehicle's build sheet. This sticker often contains detailed information about the vehicle's equipment, including the axle ratio.
- Look at the differential: Some differentials have a metal tag with the gear ratio stamped on them, or there may be a sticker on the axle itself.
- Consult your owner's manual: The owner's manual or other paperwork from the vehicle's purchase often lists the gear ratio.
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.
How do you tell what size differential you have?
Visual Differential Identification
Without an ID tag, differentials are identified by the number of cover bolts, the distinctive shape of the cover, the number of ring gear bolts, and ring gear diameter. They can also be identified by whether the center section is an integral or dropout design.
