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You cannot order a ring and pinion, a locker, a carrier or even the right axle shafts until you know two things: which axle is under the truck and what ratio it has now. Guessing from the year and model works maybe 70 percent of the time. The other 30 percent ends with a carrier that does not fit or a regear to the ratio you already had. Here are the four ways we confirm it, from easiest to most certain.
Method 1: The Factory Code
GM: the RPO sticker
GM trucks and SUVs have a Service Parts Identification sticker, usually in the glove box, sometimes on the inside of the driver door or under the trunk lid on cars. It is a grid of three-character RPO codes. The axle codes start with G:
| RPO | Meaning |
|---|---|
| GU2 | 2.73 ratio |
| GU4 | 3.08 ratio |
| GU5 | 3.23 ratio |
| GU6 | 3.42 ratio |
| GT4 | 3.73 ratio |
| GT5 | 4.10 ratio |
| G80 | Locking rear differential (Eaton automatic locker) |
Newer GM trucks added more ratio codes over the years, so if you see a G code that is not on this list, look it up against the specific model year rather than assuming. The sticker also does not tell you which axle (8.5, 9.5, 9.76, 11.5), only the ratio and whether it has a locker. Stickers also go missing or fade. On a used truck with a swapped axle, the sticker is lying to you.
Ford: the door label and the axle tag
Ford puts a two-character axle code on the Safety Compliance Certification label on the driver door jamb, in the box marked AXLE. The code changes meaning between platforms and years, so decode it against the table for your exact truck. The more useful item is the metal tag held on by a rear cover bolt. On an 8.8 or 9.75 it reads something like 3L73 or 3 55. The digits are the ratio. An L in the middle means limited slip (Traction-Lok). No L means open.
Ram and Jeep: tag or build sheet
Chrysler-family axles often have a tag on a cover bolt with the ratio stamped on it, and many have the ratio and build date stamped into the axle tube near the carrier. If the tag is gone, a dealer parts counter or an online VIN build sheet lookup will show the factory axle sales code. That only tells you what the truck left the factory with.
Method 2: Count the Bolts and Measure the Ring Gear
The code tells you ratio. It rarely tells you which axle you have. The cover bolt count and shape narrow it fast:
- 10 bolts: GM 7.5/7.625 and 8.5/8.6 (rounded cover), Ford 7.5 and 8.8 (irregular cover with a flat top edge), Chrysler 8.25, Dana 35 (oval).
- 12 bolts: GM 9.5 and 9.76 (the modern 12-bolt), Chrysler 9.25, classic GM 12-bolt car and truck.
- 14 bolts: GM 14-bolt 10.5 and AAM 11.5 (the AAM 11.5 cover is not interchangeable with the old 14-bolt).
- No rear cover at all: Ford 9-inch and other drop-out third member axles, where the whole center section unbolts from the front of the housing.
Dana axles usually have the model number cast into the housing at the top of the diff: a 30, 35, 44, 60 or 70 in the casting, plus a part number tag. Our per-axle pages, like the GM 8.5 10-bolt guide and the Ford 8.8 guide, have photos of the covers and casting marks worth comparing.
Method 3: Spin the Tire and Count
No tag, no sticker, or you think the axle has been regeared. This takes 10 minutes and a paint pen.
- Block the front wheels, put the transmission in neutral and release the parking brake. On a 4x4, put the transfer case in 2WD.
- Jack up the rear so both wheels are off the ground and on stands.
- Put a chalk mark on a tire at the 12 o'clock position and a mark on the driveshaft at the bottom.
- Turn the tire exactly one full turn, keeping the other rear tire turning the same direction with it (a helper, or it will follow on its own with a limited slip or locker). Count driveshaft turns.
The driveshaft count is the ratio. A little over three and a half turns is 3.55 or 3.73. Close to four is 3.92 or 4.10. Getting 3.73 vs 3.55 right by eye is tough, so do ten tire revolutions and count: 35.5 driveshaft turns is a 3.55, 37.3 is a 3.73.
If you can only raise one wheel and the axle is an open diff, the raised wheel turns twice as fast as it would with both wheels up. Turn it two full turns and the driveshaft count is the ratio. This does not work with a limited slip or locker, because the grounded wheel holds the carrier.
Method 4: Read the Ring Gear
When the cover is off for a fluid change, the answer is stamped on the ring gear. Look on the back face or the outer edge for a pair of numbers like 41-11 or 41 x 11. That is ring gear teeth over pinion teeth. Divide and you have the ratio:
| Stamping | Ratio |
|---|---|
| 41-15 | 2.73 |
| 40-13 | 3.08 |
| 41-12 | 3.42 |
| 39-11 | 3.55 |
| 41-11 | 3.73 |
| 47-12 | 3.92 |
| 41-10 | 4.10 |
| 37-9 | 4.11 |
| 41-9 | 4.56 |
| 39-8 | 4.88 |
Different axles use different tooth counts for the same nominal ratio, so a Dana 35 4.10 and a GM 4.10 may not be stamped the same. Divide what you see. Aftermarket gear sets also usually have the brand and part number etched near the tooth count, which tells you someone has been in there before you. That is the moment to check the backlash pattern before trusting it.
Do Not Forget Spline Count
If you are shopping for a carrier, locker or axle shafts, ratio and axle are only two-thirds of the answer. Count axle shaft splines too. With the axle shaft out, count the teeth on the end, or count the splines on the side gear visible through the carrier window. The GM 8.5 came in 28 and 30 spline, the Dana 44 in 30 and 32, the Ford 8.8 in 28, 31 and 34. Getting it wrong is the most common return we see on carriers.
Now That You Know
If you are changing ratio for bigger tires, our regear after bigger tires guide has the math. Gear sets we stock include the Ford 8.8 3.73 ring and pinion ($252.99), the GM 8.5 10-bolt 3.42 ring and pinion with master install kit ($399.99) and the Dana 35 4.88 ring and pinion ($317.99). For the setup itself, the ring and pinion setup tool kit ($188.99) has the bearing pullers and setup tools most home garages are missing.
If the gearing problem you are fighting is really an overdrive or transmission ratio issue, the transmission side lives at CoreTransmissionParts.com.
Regear It Right the First Time
Gear sets, master install kits and the setup tools to read the pattern.
Shop Gear Sets → Setup Tool KitFAQ
How do I find my rear axle ratio without a tag?
Raise both rear wheels, put the transmission in neutral, mark a tire and the driveshaft, and turn the tire one full turn while counting driveshaft turns. The count is the ratio. For accuracy, turn the tire ten times and divide the driveshaft count by ten.
What does GU6 mean on a GM sticker?
GU6 is the GM RPO code for a 3.42 axle ratio. GU5 is 3.23, GT4 is 3.73 and GT5 is 4.10. G80 means the axle has the factory locking differential.
What does the L mean on a Ford axle tag?
On a Ford axle tag such as 3L73, the L means the axle has a limited slip (Traction-Lok) differential. The digits are the ratio, so 3L73 is a 3.73 limited slip and 3 55 without an L is a 3.55 open axle.
Where is the gear ratio stamped on a ring gear?
Usually on the back face or outer edge as two numbers, ring gear teeth then pinion teeth, such as 41-11. Divide the first by the second to get the ratio: 41 divided by 11 is 3.73.