How to Determine the Final Gear Ratio in a BMW

1. Raise the vehicle using a floor jack, and place it securely on jack stands if necessary.
2. Locate the differential between the two rear tires. Depending on the year and model, the stamp may be on the either side or on the rear of the differential. Look for a flat, smooth area next to a bolt, with a series of letters and numbers stamped on it.
3. Clean the stamping using a rag and wire brush, if necessary. Depending on the age and use of the vehicle, a significant amount of grime may have accumulated.
4. Decode the stamping. It will be comprised of eight characters. The first is a letter, most likely an N or R, which designates an open differential. The letter S signifies a limited-slip differential. After that will be three digits, such as 345. This indicates a final drive ratio of 3.45:1. After that will be four digits, such as 0722, which refer to the part number of the differential.