How do you tell if you have a 8.5 or 8.6 rear end?

Quick answer: identify the axle by reading the stamped code or tag on the housing, or physically measure the ring gear diameter. If the ring gear measures about 8.50 inches, you have an 8.5-inch axle; if it’s about 8.60 inches, you have an 8.6-inch axle.

In practice, distinguishing between these two sizes combines paperwork and hands-on inspection. The most reliable clues are axle identification codes stamped on the housing or a sticker/tag on the third member, plus a direct measurement of the ring gear once you have access to the gear inside the differential. This guide explains the steps, what to look for, and how the difference matters for parts like gears, bearings, and carriers.

What the numbers mean

The terms 8.5 and 8.6 refer to the nominal diameter of the ring gear in inches used by particular rear-axle families. The size affects which gears, carriers, seals, and third members will fit, so correctly identifying it helps you source compatible parts and avoid mismatches.

Ways to identify your axle

Use these steps to confirm your axle size. They combine visible marks and a direct measurement approach so you can verify even without records.

1. Locate the axle identification tag or stamping: Many axles have a metal tag or stamped codes on the differential housing or axle tube. Look for numbers or letters that indicate the axle family, ring gear size, or a code you can cross‑reference with a parts catalog or dealer database.


2. Inspect the differential cover or carrier for part numbers or codes: A catalog lookup using any markings you find can reveal the ring-gear size and axle family.


3. Access and measure the ring gear: If you can remove the differential cover and the carrier, measure the outer diameter of the ring gear teeth with calipers. A ring gear near 8.50 inches points to an 8.5-inch axle, while a ring gear near 8.60 inches points to an 8.6-inch axle.


4. Check casting numbers on the housing and third member: Some manufacturers stamp casting numbers that can be cross-referenced to identify the axle size and production window.


5. Cross-check with the vehicle’s VIN or build data: Your vehicle’s VIN or production sheet from the manufacturer can indicate the original axle specification for that model and year.

Notes and caveats: Some owners swap third members or ring gears during repairs or upgrades, which can blur the true axle size if you’re relying on appearance alone. Always verify with multiple indicators when possible, and consider professional help if you’re unsure.

Practical tips and pitfalls

What to watch for when you're trying to identify an 8.5" vs 8.6" rear end:

• Aftermarket or remanufactured axles may carry different sizes even on the same vehicle, so don’t rely on a single clue.


• Documentation (owner manuals, build sheets, or dealer records) is often the fastest route to accuracy.


• If you’re planning to order gears or a third member, know that incorrect sizing can cause fitment issues or improper gear engagement.

In case of conflicting signals, prioritize verifiable data: an official axle tag or a direct ring-gear measurement, then consult dealer catalogs or a professional installer for confirmation.

What to do if you’re still unsure

If you can’t locate reliable markings or you can’t safely access the ring gear, bring the vehicle to a qualified shop or parts dealer. They can cross-reference the VIN, inspect the axle, and confirm whether you’re dealing with an 8.5-inch or 8.6-inch rear end, ensuring you buy the correct parts the first time.

Summary

Identifying an 8.5-inch versus an 8.6-inch rear axle comes down to verifying an official axle tag or stamping and, if needed, directly measuring the ring gear diameter. Always corroborate with multiple indicators—tag codes, casting numbers, VIN data, and what a trained technician observes during inspection—to avoid part mismatches and drivetrain issues.

Bottom line: start with documentation, then confirm with a careful measurement of the ring gear or a definitive tag reading. That combination yields the most reliable determination of whether your rear end is 8.5" or 8.6".

How to identify an 8.5 rear end?

The 7.5 has a more squared cover, while the 8.5 has a round cover. The 8.5 rear end also has axle tubes that step down from 3" to 2.75" coming out of the pumpkin. 8.5s were available from 1995-05 in all ZR2s. Starting in late '97 4.3 5-speed trucks began to be equipped with the 8.5 rear, whether it was 2wd or 4wd.

What vehicles came with an 8.5 rear end?

You can find a 10-bolt 8.5-inch differential in any of these cars or trucks:

• 1970-'81 Chevrolet Camaro.
• 1970-'81 Pontiac Firebird.
• 1971-'76 Chevrolet Impala and Caprice.
• 1971-'81 Buick Centurion, Electra, Electra 225, and LeSabre.
• 1971-77 Pontiac Grand Prix.
• 1971-'77 Oldsmobile Cutlass.

How do I tell what rear end I have?

You can identify your rear end by checking the RPO codes or the vehicle's door sticker, looking for an ID tag on the differential cover, or performing a visual inspection. The most reliable method is checking codes, but visual identification is a good alternative if the codes are missing. 
This video shows how to check your car's differential gear ratio: 59sCatfish MotorsportsYouTube · Jan 11, 2024
Check vehicle codes

• Driver's side door sticker: Look for a sticker with a code that specifies the axle ratio or type. 
• Glove box: Many vehicles have a sticker with RPO (Regular Production Option) codes in the glove box or owner's manual. Find the code related to the axle and look up what it means. 
• VIN: Use your Vehicle Identification Number (VIN) at a dealership or an online VIN decoder to get a full list of your vehicle's original equipment. 

Inspect the differential

• Look for a metal tag: On the differential cover, there may be a small metal tag with the gear ratio and other identifying information stamped on it. 
• Check the differential housing: Look for casting numbers or stamped information on the housing itself, though these can be hard to read due to dirt and age. 
• Visually identify by shape and bolts: Each rear end model has a unique design. You can identify many differentials by the number of bolts on the cover, the shape of the cover, and the number of ring gear bolts. 
  • For example, an 8-inch Ford housing is rounded with two dimples, while a 9-inch housing has a raised center with two flat spots. 

Perform a drive shaft rotation test 

• If the vehicle is a conventional or open differential: Jack up the rear so both wheels are off the ground and the vehicle is in neutral. Spin the driveshaft and count how many times it turns for one full revolution of a rear wheel. The ratio will be approximately the driveshaft rotations divided by two. For example, if the driveshaft turns 20 times, the ratio is 20÷2=4.1020 divided by 2 equals 4.1020÷2=4.10.
• If the vehicle has a locking differential: If one wheel is held stationary, both wheels will spin in the same direction. To get the gear ratio, you'll need to spin the driveshaft and count how many times it turns for one wheel revolution, and then multiply that number by two. 

This video demonstrates how to determine what gears you have without removing the differential cover: 58sBent Motorsports OffroadYouTube · Sep 22, 2023

How do I tell what size rear differential I have on my Chevy?

Identify your GMC or Chevrolet rear end gear ratio by the RPO Codes, usually located on the placard labeled “Service Parts Identification” inside the drivers door panel or glove box. Note that using the RPO code for parts identification assumes that the rear end has not been previously modified!!