How do I know if I have a 8.5 or 8.6 rear end?

If you're unsure whether your vehicle has an 8.5 or 8.6 rear axle, there are a few ways to determine the exact model. This information is important for selecting the right replacement parts or accessories for your car or truck.

Identifying the Rear Axle Type

The easiest way to identify your rear axle type is to look for the axle identification tag. This tag is usually located on the rear axle housing or differential cover and will clearly state whether it's an 8.5 or 8.6 model. If the tag is missing or unreadable, you'll need to inspect the axle housing itself.

• An 8.5 rear axle will have a housing diameter of 8.5 inches.


• An 8.6 rear axle will have a housing diameter of 8.6 inches.

You can measure the diameter of the axle housing to determine the model. Another way is to look at the brake components - an 8.5 axle will have smaller brake components than an 8.6.

Why the Difference Matters

Knowing whether you have an 8.5 or 8.6 rear axle is important because the two models use different parts and components. Attempting to install the wrong parts can lead to fitment issues or even damage to your vehicle. For example, the ring and pinion gears, differential cover, and brake components are all specific to the 8.5 or 8.6 axle type.

Properly identifying your rear axle is the first step to ensuring you purchase the correct replacement parts or upgrades for your vehicle. Taking the time to inspect the axle housing or locate the identification tag can save you time and money in the long run.

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 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!!

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