The axle identification number is located either on a metal tag (attached to the rear of the differential) or stamped onto the front side (usually right) of the axle tube, about three inches outboard from the differential cover. For servicing the rear axle, the third letter must be known, which is the manufacturers identity. The codes are: B for Buick, C for Buffalo, G for Chevrolet-Gear and Axle, K or M for GM of Canada, O for Oldsmobile, P for Pontiac and W for Warren. The rear axle identification number must be known to order the exact replacement parts.
On all late model axle assemblies, the axle ratios are designed to meet emission standards for areas of operation. Depending on the performance options on your vehicle original equipment ratios may start as low as 2.29 and go as high as 3.42. Possible factory rear axle codes include the following codes and ratios.
An axle ratio is obtained by dividing the number of teeth on the drive pinion gear into the number of teeth on the ring gear. For instance, on a 4.11 ratio, the driveshaft will turn 4.11 times for every turn of the rear wheel. The most accurate way to determine the axle ratio is to drain the differential, remove the cover and count the number of teeth on the ring and pinion.
The axle ratio is also identified by the axle serial number prefix on the axle; the axle ratios are listed in dealer's parts books according to prefix number. Some axles have a tag on the cover.
If the identification tag is missing, or does not have the gear ratio stamped on it or the axle is not original and cannot be identified, there is a quick way to determine the axle ratio:
- Block the front wheels and place the transmission in
- Raise and support the rear of the vehicle safely using jackstands.
- Mark the inside of the wheel with chalk or other suitable marker.
- Mark the pinion yoke at the housing.
- With transmission in neutral, turn the driveshaft (or pinion) and count the number of turns it takes to turn the wheel one complete revolution.
More accuracy can be obtained by going more than one tire revolution and dividing the result by the number of tire rotations.
You now have an estimated axle ratio, examples being; 2 1 / 2 turns of the pinion to 1 turn of the wheel equals about a 2.50 ratio. 3 1 / 4 turns of the pinion to 1 turn of the wheel equals about a 3.25 etc. Remember this is just an estimated ratio and the actual ratio may be slightly different. The actual ratio is usually necessary when ordering replacement parts.