You are standing at a rear axle in a salvage yard and need to confirm the gear ratio before you pull it. The axle tag may be missing, corroded beyond legibility, or the donor vehicle’s door sticker may be inaccessible. Installing an axle with the wrong gear ratio creates a range of problems from subtle to serious: mismatched speedometer readings, abnormal transmission shift points, tire wear from ratio mismatch with other axles on a four-wheel-drive vehicle, and drivability issues when the ratio change is significant enough to affect engine operating RPM at speed.
Getting this right before you pull saves you the cost of a second trip, a wasted core charge, and the diagnostic time to figure out why the swap does not behave correctly. There are four distinct methods for confirming an axle ratio in the junkyard, and each applies to a different situation depending on what information is available on the donor vehicle. This guide covers all four in the order you should attempt them: tag reading first, door sticker second, rotation count third, tooth count last.
This guide also covers manufacturer-specific tag formats, GM RPO codes for axle ratio identification, and the common axle ratios found in specific platforms so you can evaluate plausibility before committing to a pull.
Why the Ratio Must Be Confirmed Before You Pull
Rear axle gear ratio is a fundamental drivetrain specification that affects every aspect of how the vehicle drives. The ratio determines how many times the driveshaft must rotate to produce one rotation of the rear wheels. A ratio of 3.73 means the driveshaft turns 3.73 times for every single rotation of the wheel. A 4.10 ratio turns the driveshaft 4.10 times per wheel revolution. The difference between these ratios is enough to change fuel economy, towing capacity feel, highway cruising RPM, and transmission shift programming behavior.
In a four-wheel-drive or all-wheel-drive vehicle, a mismatched ratio between the front and rear axles creates constant binding in the drivetrain when all-wheel-drive is engaged, which damages differentials, transfer cases, and axle shafts over time. Even in a two-wheel-drive swap context, installing an incorrect ratio without knowing what you have installed makes accurate troubleshooting impossible.
The ratio that appears most commonly in a salvage yard for any popular platform is not always the ratio that was original to the specific trim level you are replacing. Performance trims typically had numerically higher ratios (3.73, 4.10) while base and economy trims had lower ratios (2.73, 3.08). Grabbing the first axle from the right platform without confirming the ratio is the most common cause of post-install surprises.
Method 1: Read the Axle Tag
The axle identification tag is a small metal plate, typically aluminum or steel, riveted to one of the differential cover bolts. It is the first and fastest source of ratio information when it is present and legible. The tag location and format vary by manufacturer.
Ford Axle Tags (8.8-inch, 8.8 IRS, 9-inch, Sterling 10.25)
Ford rear axle tags are typically riveted to the driver’s side of the differential cover or to an axle tube. The tag format uses a combination of letters and numbers. On Ford 8.8-inch axles (the most common Ford rear axle), the tag may show the ratio as a decimal (3.73, 3.55, 4.10) directly or as a coded sequence. The ratio is printed or stamped in clear numeric form on tags that are still legible.
Ford also identifies axle ratios through an axle code on the door jamb certification sticker. The axle code is a two-digit code (such as “26” for 3.55 or “28” for 3.73 on many F-150 and Mustang applications) that appears in the vehicle certification label on the driver’s door jamb. A cross-reference chart translating Ford axle codes to ratios is available for most Ford platforms through online resources and enthusiast communities.
GM Axle Tags (10-bolt, 12-bolt, 8.5, 8.6, 9.5, 14-bolt)
GM rear axle tags are riveted to one of the differential cover bolts and typically show the ratio in decimal format (3.73, 3.42, 4.11) with additional codes for build date and plant. The tag on a GM 10-bolt or 12-bolt axle is usually on the passenger side of the cover. The ratio is the most prominent numeric information on the tag when it is present. On many GM trucks and SUVs, the tag clearly reads the ratio: “3.73,” “4.10,” etc.
Chrysler and Mopar Axle Tags (8.25, 8.75, 9.25, 11.5 AAM)
Chrysler rear axle identification tags are located on the driver’s side axle tube, near the differential housing, rather than on the cover bolts in many applications. The Chrysler tag format includes a gear ratio designation that may appear as “3.92,” “3.55,” or similar. Some Mopar applications use a bar code tag rather than a stamped metal tag; the bar code contains encoding that requires a decoder or manufacturer reference to translate.
Method 2: Read the Door Jamb Sticker and VIN
The driver’s door jamb certification label contains the vehicle’s production specifications in coded form. This label is intact on most salvage yard vehicles unless the door has been removed or the label itself has been damaged. The label typically shows month and year of manufacture, GVWR ratings, tire size, and a series of production codes that include the axle ratio.
Reading GM RPO Codes From the Door Jamb or Glove Box
GM vehicles from the 1970s through the 2000s carry a sticker either in the driver’s door jamb or in the glove box that lists all Regular Production Option (RPO) codes for that specific vehicle. Each RPO code is a three-character alphanumeric code representing a specific factory-installed option or specification. The RPO code list is one of the most reliable ways to determine the original axle ratio of any GM vehicle in a salvage yard.
| GM RPO Code | Axle Ratio | Notes |
| GU2 | 2.56 | Economy ratio; uncommon in performance applications |
| GU4 | 3.08 | Light truck and passenger car economy ratio |
| GU5 | 3.23 | Uncommon; found in some light truck applications |
| GU6 | 3.42 | Common truck and SUV mid-range ratio |
| GT4 | 3.73 | Common performance and towing truck ratio |
| GT5 | 4.10 | Higher ratio; trucks and off-road applications |
| G76 | 3.73 | Passenger car performance ratio; Camaro, Corvette applications |
| G92 | 3.42 | Passenger car mid-range ratio |
| GX9 | 3.73 | Heavy duty axle with 3.73 ratio |
| GX4 | 4.10 | Heavy duty axle with 4.10 ratio |
| G80 | Positraction (limited slip) | Identifies a posi rear end but not the ratio; always paired with a ratio code |
The G80 code is important to note because it indicates a positraction (limited slip) differential without identifying the gear ratio. It always appears alongside a ratio code. A vehicle showing G80 and GT4 has a 3.73 posi rear axle. A vehicle showing G80 without any ratio code has an incomplete or damaged sticker; the rotation count or tooth count method is then required.
Method 3: The Rotation Count Method
The rotation count method is the most reliable physical verification of axle ratio and can be performed without any tag, sticker, or documentation. It works by measuring how many times the driveshaft rotates for each complete rotation of a rear tire, which directly reveals the gear ratio.
When This Method Applies
Use the rotation count method when the axle tag is missing or illegible, the door jamb sticker is absent or damaged, and the RPO sticker cannot be located or read. It requires that the vehicle’s transmission can be placed in neutral and that one rear tire can be rotated manually.
How to Perform the Rotation Count
Step 1: Ensure the vehicle is safe and stable. Chock the non-rotating wheels firmly. The vehicle must not move during the procedure.
Step 2: Place the transmission in neutral. For a vehicle without a key, use the shift lock override to move the selector to neutral, as described in service documentation for the specific transmission. For a manual transmission, simply disengage the gear selector.
Step 3: Block or hold the opposite rear tire firmly to prevent it from rotating. On an open differential (non-posi), rotation of one tire causes the other to rotate in the opposite direction; blocking the opposite tire forces all driveshaft rotation to translate through the side you are measuring.
Step 4: Mark a reference point on the driveshaft (chalk or tape) and a reference point on the tire you are rotating (chalk mark at the 6 o’clock position on the sidewall).
Step 5: Rotate the marked tire exactly TWO full revolutions while counting how many times the driveshaft reference point passes its starting position.
Step 6: Divide the driveshaft rotation count by 2 (because you rotated the tire twice). The result is the gear ratio. If the driveshaft rotated 7.46 times during two tire revolutions, the ratio is 3.73. If it rotated 8.20 times, the ratio is 4.10.
| Tire Rotations Measured | Driveshaft Rotations (2 tire turns) | Calculated Ratio |
| 2 tire turns | 5.46 driveshaft turns | 2.73:1 |
| 2 tire turns | 6.16 driveshaft turns | 3.08:1 |
| 2 tire turns | 6.84 driveshaft turns | 3.42:1 |
| 2 tire turns | 7.10 driveshaft turns | 3.55:1 |
| 2 tire turns | 7.46 driveshaft turns | 3.73:1 |
| 2 tire turns | 7.82 driveshaft turns | 3.91:1 |
| 2 tire turns | 8.20 driveshaft turns | 4.10:1 |
| 2 tire turns | 9.12 driveshaft turns | 4.56:1 |
Measuring two tire rotations rather than one reduces counting error by averaging the measurement. Count to the nearest quarter turn of the driveshaft and divide by 2. The result will be close to but rarely exactly a standard ratio number due to counting imprecision; match your result to the nearest standard ratio in the table above. A result of 7.40 is 3.73, not 3.70. There is no standard 3.70 ratio; round to the nearest published ratio.
Method 4: Count the Ring and Pinion Gear Teeth
Counting ring and pinion gear teeth is the most precise method of determining axle ratio and does not require the transmission to be in neutral or any tire rotation. It requires that the differential cover be removed or already absent, which is common on heavily stripped salvage yard vehicles.
The ring gear is the large gear visible inside the differential housing when the cover is removed. The pinion gear is the smaller gear at the front of the differential, connected to the driveshaft. The ratio is calculated by dividing the number of ring gear teeth by the number of pinion gear teeth.
How to count: use a paint marker, chalk, or a piece of tape to mark the starting tooth on the ring gear. Count every tooth around the full circumference, marking every tenth tooth to maintain accuracy. Then count the pinion teeth. Divide the ring tooth count by the pinion tooth count. The result is the axle ratio.
Example: a ring gear with 41 teeth and a pinion gear with 11 teeth gives 41 divided by 11 equals 3.727, which rounds to 3.73. A ring gear with 45 teeth and an 11-tooth pinion gives 45 divided by 11 equals 4.09, which rounds to 4.10. This method is definitive regardless of tags, stickers, or any other documentation.
Manufacturer-Specific Axle Identification Reference
| Manufacturer / Axle | Tag Location | Tag Format | Door Sticker Code |
| Ford 8.8 (most common Ford rear axle) | Passenger side of differential cover, on a cover bolt | Decimal ratio stamped (3.73, 4.10, etc.) or coded sequence | Two-digit axle code in certification label; cross-reference online by model year |
| Ford 9-inch (classic; Fox Body, older trucks) | Driver’s side axle tube near housing | Build date and ratio coding; often missing on older examples | Pre-computer era; VIN decode and enthusiast cross-reference required |
| GM 10-bolt (passenger car and light truck) | Passenger side of differential cover | Ratio in decimal form; build date code | RPO codes in glove box or door jamb sticker |
| GM 12-bolt (classic muscle car axle) | Cover bolt, typically driver’s side | Stamped coding; enthusiast guides decode full format | Pre-computer era; casting number and enthusiast database required |
| GM 8.5 and 8.6 (modern trucks, SUVs) | Passenger side cover bolt | Ratio in decimal on tag; build date | RPO codes on door jamb or glove box sticker |
| GM 14-bolt full-float (heavy trucks) | Driver’s side axle tube | Ratio and build info stamped | RPO codes; GT4, GT5, GX9 most common |
| Chrysler 8.25 (common compact truck axle) | Driver’s side axle tube near diff housing | Ratio code; may require Chrysler cross-reference guide | Option code sticker; varies by platform and year |
| Chrysler 9.25 (full-size truck and SUV) | Driver’s side tube or cover area | Decimal ratio or coded; check Mopar resources | Build sheet sticker in vehicle if present |
| Dana 44 (Jeep, Ford, and others) | Front or rear axle tube, often driver’s side | Build date, ratio, and plant code; Dana-specific format | Option code sticker; Jeep uses well-documented decoding systems |
For any axle where the tag is missing and the door sticker is absent, the rotation count or tooth count method is the definitive verification. Never guess or assume based on what the vehicle “should” have had as original equipment. Salvage yards regularly contain vehicles that have had axle swaps from previous owners, meaning the axle currently installed may not match the original specification for that trim level.
Common Rear Axle Ratios and What They Indicate
| Ratio | Typical Application | What It Means |
| 2.73 | Economy trucks, base passenger cars from 1980s to 1990s | Tall ratio for highway fuel economy; poor towing and acceleration |
| 3.08 | Economy trucks and SUVs; some passenger cars | Mild fuel economy bias; acceptable for light towing |
| 3.23 to 3.27 | Mid-range trucks and passenger performance cars | Balanced ratio; moderate towing and reasonable economy |
| 3.42 | Common truck, SUV, and muscle car ratio | Good all-around ratio; competent towing; the most common ratio in many platforms |
| 3.55 | Performance passenger cars; medium-duty towing trucks | Slightly aggressive; better acceleration; common in V8 Mustangs |
| 3.73 | Most common performance and towing ratio in trucks and muscle cars | Excellent towing; strong acceleration; compromises highway RPM slightly |
| 3.91 to 4.10 | Performance trucks, off-road builds, muscle car performance applications | Aggressive; high RPM at highway speed; strong off-the-line performance |
| 4.30 to 4.56 | Jeep off-road, heavy-duty tow rigs, racing applications | Very tall gearing; not suitable for highway use without overdrive gearing consideration |
| 4.88 and numerically higher | Dedicated off-road; crawl ratio applications | Extreme low end; specific purpose only |
When a rotation count or tooth count produces a result that does not match any standard ratio in this table, re-check your count before concluding the axle has a non-standard ratio. Counting errors are common because maintaining focus through 40+ ring gear teeth and fractional driveshaft rotations is challenging. A second count on the same axle that produces the same result confirms the ratio; a different result on the second count indicates a counting error on one of the attempts.
Frequently Asked Questions
How do I find the gear ratio on a junkyard axle?
Use four methods in order. First, look for the axle tag, a small metal plate on a differential cover bolt or axle tube, which typically shows the ratio in decimal form. Second, read the driver’s door jamb certification sticker, which shows axle ratio codes that cross-reference to specific ratios. Third, use the rotation count method: place the transmission in neutral, block one rear tire, rotate the other tire twice, count driveshaft rotations, and divide by 2. Fourth, count ring and pinion gear teeth and divide ring teeth by pinion teeth.
Where is the axle ratio tag on a Ford 8.8 rear end?
The axle identification tag on a Ford 8.8-inch rear axle is typically riveted to one of the differential cover bolts, usually on the passenger side. It may also appear on the driver’s side in some production years. The tag shows the ratio in decimal format (3.73, 4.10, etc.) or in a coded sequence that Ford-specific cross-reference charts can decode. Ford also identifies axle ratios through a two-digit axle code on the driver’s door jamb certification sticker.
What GM RPO codes identify rear axle ratios?
GM RPO codes for rear axle ratios include GU4 (3.08), GU6 (3.42), GT4 (3.73), GT5 (4.10), G76 (3.73 for passenger cars), G92 (3.42 for passenger cars), GX9 (3.73 heavy duty), and GX4 (4.10 heavy duty). G80 identifies a positraction (limited slip) differential but not the specific ratio; it always appears alongside a ratio code. These codes are found on the RPO sticker in the glove box or on the driver’s door jamb of GM vehicles.
How do I use the rotation count method to find axle ratio?
Place the transmission in neutral and block one rear tire to prevent rotation. Mark the driveshaft and the free rear tire with chalk. Rotate the free tire exactly two full revolutions and count how many complete rotations the driveshaft makes. Divide the driveshaft count by 2 to get the ratio. For example, if the driveshaft rotates 7.46 times during 2 tire revolutions, the ratio is 3.73. Use the count of two tire revolutions rather than one to reduce measurement error.
How do I calculate gear ratio by counting teeth?
Remove or access the differential cover to expose the ring gear. Count all the teeth on the ring gear, marking every tenth tooth to maintain accuracy. Count the teeth on the pinion gear at the front of the differential. Divide the ring gear tooth count by the pinion gear tooth count. The result is the axle ratio. A ring gear with 41 teeth and a pinion with 11 teeth equals 41 divided by 11, which is 3.73.
How do I know if a junkyard axle has positraction?
Positraction (limited slip differential) can be identified several ways. GM vehicles with posi have the G80 RPO code on their sticker. Physically, place the axle in neutral and attempt to rotate one tire while holding the other stationary; in a posi unit, the stationary tire will also want to rotate in the same direction, requiring force to hold it. In an open differential, you can rotate one tire with essentially no resistance while the other remains still. Some axle tags also have codes indicating posi that can be cross-referenced through manufacturer documentation.
What is the most common rear axle ratio found in junkyard trucks?
The 3.73 ratio is one of the most commonly found ratios in junkyard trucks and SUVs from American manufacturers because it was a popular factory option for towing and performance applications. The 3.42 ratio is similarly common as the middle-tier option in many trucks. The 4.10 ratio is less common but regularly found in performance and heavy-duty trim vehicles. The 2.73 and 3.08 ratios appear in base and economy trim vehicles and are less frequently sought by parts buyers.
Can a junkyard axle have a different ratio than what the car’s trim level originally specified?
Yes, and this is common. Previous vehicle owners may have performed axle swaps to change gear ratios for towing, performance, or other reasons. A vehicle that originally came with a 3.08 economy ratio may have had a 3.73 axle installed. The axle currently in the vehicle may not match what the door sticker or VIN decode says it should have. The physical verification methods, rotation count and tooth count, confirm the actual ratio regardless of what any documentation suggests it should be.
The Bottom Line
Confirming a rear axle ratio before pulling is a 10 to 20-minute investment that prevents a frustrating incorrect pull and the cost of a second trip. The four methods, axle tag, door sticker, rotation count, and tooth count, cover every situation you are likely to encounter in a self-service yard. Start with the fastest (tag reading) and proceed to the most definitive (tooth count) only when earlier methods are unavailable or produce ambiguous results.
The rotation count method is the one worth memorizing completely before any junkyard axle search. It requires no special tools, works on any axle regardless of how stripped the donor vehicle is, and produces a directly readable result without any cross-reference documentation. Two tire rotations, divided by the driveshaft count, equals your ratio. That calculation is all you need.
The manufacturer-specific tables in this guide cover the tag formats, RPO codes, and common ratios for the platforms most frequently found in American salvage yards. For platforms not covered here, enthusiast forums and online communities for your specific make and model maintain detailed axle identification guides developed by owners who have done exactly what you are doing.