1. Check the Vehicle's Identification Number (VIN) Decoder:
* Online VIN decoders: Many websites offer free VIN decoding services. Enter your Suburban's VIN, and it *might* reveal the axle ratio (gear ratio). However, this information isn't always included. This is your best bet for a quick non-invasive check.
2. Look for a Tag on the Differential:
* Differential cover: The rear differential cover (the metal cover on the rear axle housing) often has a tag or stamp indicating the gear ratio. You'll likely need to get under the vehicle to see this. The tag might be small and rusty, so a flashlight and possibly a wire brush will be helpful. The ratio will be expressed as a number like 3.73, 4.10, etc.
3. Check the Owner's Manual (if you have it):
* While unlikely, your original owner's manual *might* list the gear ratio. It's not a common piece of information included in these manuals.
4. Consult a Repair Manual:
* A repair manual specific to your 1991 Chevy Suburban (year and model are crucial) will sometimes specify the gear ratios for different options. Chilton and Haynes manuals are common sources.
5. Look at the Build Sheet (if you can find it):
* If you're lucky, the original build sheet for your Suburban might still exist. This would definitely list the gear ratio. This is a long shot though.
6. Jack Up the Vehicle and Count Rotations (Least Accurate Method):
* WARNING: Safety is paramount. Use jack stands, chock the wheels, and follow safe jacking procedures.
* This is the least accurate and most involved method. Raise the rear of the vehicle. Mark a spot on the driveshaft and a spot on the tire. Rotate the tire one full revolution and count how many times the driveshaft rotates. The number of driveshaft rotations is the numerator, and the number of tire rotations is the denominator. This gives you the ratio. You'll need to account for the transmission being in neutral. This method is prone to error.
Important Considerations:
* Front vs. Rear: If your Suburban has 4-wheel drive, it has a front and a rear differential. The rear axle ratio is usually the one people are most interested in.
* Multiple Ratios: It's possible that different gear ratios were used across different models and options in 1991.
Start with the VIN decoder and checking the differential cover. These are the easiest methods. If those fail, you'll likely need a repair manual or to resort to the less accurate rotation method. Remember safety first when working under your vehicle.
