Staring at a two-character code on your door jamb and wondering what it actually means? Ford rear axle codes tell you everything about your gear ratio, differential type, and axle assembly — if you know how to read them. This guide breaks down every code, tag, and physical test you need to identify your Ford’s rear axle with confidence. Stick around — the physical rotation test alone is worth the read.
What Ford Rear Axle Codes Actually Tell You
Ford rear axle codes aren’t random. Each code identifies three things at once: your gear ratio, your differential carrier type (open, limited-slip, or electronic locking), and indirectly, your axle’s load rating.
You’ll find the primary axle code on the Safety Compliance Certification Label — that’s the sticker on your driver-side B-pillar or the rear edge of the driver’s door. Look toward the bottom of the label, below the barcode. You’ll see the word “AXLE” printed above a two-character alphanumeric code. Ford’s own support page confirms this is the fastest way to identify your axle ratio and differential type.
Think of it as your axle’s birth certificate — printed right there in the door frame.
Ford F-150 Rear Axle Codes (2010 and Newer)
Modern F-150s run three differential types: standard open, Traction-Lok limited-slip, and electronic locking differentials (ELD). The ELD uses an electromagnetic actuator to lock both rear wheels together, pushing equal torque to each side.
Here’s every door-label code for post-2010 F-150 trucks:
| Door Label Code | Gear Ratio | Differential Type |
|---|---|---|
| 15 | 3.15:1 | Open Differential |
| L5 | 3.15:1 | Electronic Locking |
| 27 | 3.31:1 | Open Differential |
| L3 | 3.31:1 | Electronic Locking |
| 19 | 3.55:1 | Open Differential |
| H9 | 3.55:1 | Traction-Lok Limited-Slip |
| L9 | 3.55:1 | Electronic Locking |
| 26 | 3.73:1 | Open Differential |
| B6 | 3.73:1 | Traction-Lok Limited-Slip |
| L6 | 3.73:1 | Electronic Locking |
| L4 | 4.10:1 | Electronic Locking |
Notice the pattern: codes starting with L always mean electronic locking. That’s a handy shortcut when you’re scanning the label quickly.
Ford Super Duty Axle Codes (F-250, F-350, F-450, F-550 Post-2011)
Super Duty trucks carry heavier axles to handle serious payload and towing numbers. The code structure shifts slightly here — these configurations include open, limited-slip, and electronic locking options scaled for heavy-duty use.
| Door Label Code | Gear Ratio | Differential Type |
|---|---|---|
| 31 | 3.31:1 | Open Differential |
| 3H | 3.31:1 | Electronic Locking |
| 35 | 3.55:1 | Open Differential |
| 3J | 3.55:1 | Electronic Locking |
| 37 | 3.73:1 | Open Differential |
| 3E | 3.73:1 | Electronic Locking |
| 3L | 3.73:1 | Limited-Slip |
| 41 | 4.10:1 | Open Differential |
| 4E | 4.10:1 | Electronic Locking |
| 43 | 4.30:1 | Open Differential |
| 4L | 4.30:1 | Limited-Slip / Locking |
| 48 | 4.88:1 | Open Differential |
The deeper 4.30:1 and 4.88:1 ratios show up on trucks spec’d for maximum towing grunt — think diesel-powered F-350s pulling fifth-wheel trailers.
Ford Mustang Rear Axle Codes
Performance cars get their own code set. The Mustang door label uses letter-pair codes rather than number-based ones:
| Door Label Code | Gear Ratio | Differential Type |
|---|---|---|
| CC | 2.73:1 | Open Differential |
| YY | 3.15:1 | Open Differential |
| GG | 3.31:1 | Limited-Slip / Locking |
| DD | 3.55:1 | Limited-Slip / Locking |
The 3.55:1 DD code is the one Mustang owners typically want to see — it’s the performance-oriented ratio that keeps the rear wheels spinning at the right time.
How to Read the Physical Differential Cover Tag
The door label gives you the factory spec. The metal tag bolted to your differential cover confirms what’s actually in the axle right now — especially useful after a used-parts swap or a rebuild.
These stamped metal tags contain two lines of alphanumeric text:
Top Line:
- First five characters = axle model designator
- Four-character date code follows (e.g., 8A05)
- First digit = year of manufacture (8 = 1998 or 2008)
- Second character = month (A = January, B = February, C = March, and so on through the alphabet)
- Last two digits = day of the month
Bottom Line:
- First three digits = gear ratio
- Open differential: numbers stamped with a space (e.g., 3 55 = 3.55:1 open)
- Traction-Lok limited-slip: letter L replaces the decimal (e.g., 3L55 = 3.55:1 limited-slip)
- Next digits = ring gear diameter in inches (88 = 8.8-inch, 9 = 9-inch, 105 = 10.5-inch)
- Remaining characters = factory plant code
On trucks built from the late 1990s onward, Ford often supplements or replaces the metal tag with a white vinyl barcode decal wrapped around the passenger-side axle tube. That decal shows an engineering part number and a five-character axle code — useful for ordering replacement parts when the metal tag is gone.
How to Physically Verify Your Gear Ratio Without Any Tags
Tags get painted over. Decals peel off. When both are missing, the manual rotation test gives you a reliable field identification without any tools beyond a floor jack and chalk.
Step 1: Chock the front wheels. Jack the rear axle off the ground and support it on jack stands. Shift into neutral.
Step 2: Mark the driveshaft lengthwise with chalk or tape. Mark the inside sidewall of one rear tire.
Step 3 — Check your differential type first:
- Both rear wheels spin the same direction → you have a Traction-Lok limited-slip
- The opposite wheel spins in reverse → you have an open differential
For a limited-slip differential:
Rotate the marked tire exactly one full revolution. Count the driveshaft rotations:
- ~3.08 rotations = 3.08:1 ratio
- Exactly 3.5 rotations = 3.50:1 or 3.55:1 ratio
- ~4.1 rotations = 4.10:1 ratio
For an open differential:
Hold one wheel completely still. Rotate the free wheel two full revolutions. The number of driveshaft rotations equals the gear ratio.
If the tag says limited-slip but both wheels don’t turn together, the internal clutch packs are worn out and need rebuilding — the ratio is still valid, but the differential isn’t functioning correctly.
Identifying Your Ford Axle Type: Banjo vs. Salisbury
Before you decode any tag, it helps to know which axle family you’re dealing with. Ford uses two fundamentally different axle housing designs:
Removable Carrier (Banjo/Dropout):
- Fully welded steel housing with no rear cover plate
- All differential components bolt into a front-loading cast iron third member
- Examples: Ford 8-inch, Ford 9-inch
- The 9-inch sits at roughly 3,600 lbs max load capacity and is legendary for its low pinion placement and third bearing support
Integral Carrier (Salisbury):
- Cast iron center section with steel tubes pressed and welded in
- Rear cover plate bolts off for internal access
- Examples: Ford 7.5-inch, 8.8-inch, Super 8.8, Sterling 10.25-inch, 10.5-inch, 11.6-inch
- The 8.8-inch handles up to 3,800 lbs and is the most common Ford rear axle on the road today
The 8.8-inch axle trades some gear contact patch for a higher pinion location — that reduces internal friction and improves fuel economy compared to the 9-inch, while the stronger housing compensates for the reduced mesh area.
Decoding Dana Spicer Axles in Heavy Ford Trucks
F-350, F-450, and F-550 dual rear wheel trucks often use Dana Spicer commercial axles — Dana 80, M275, and M300. These axles don’t use Ford’s internal code system at all.
Instead, Dana Spicer axles stamp a Bill of Material (BOM) number directly into the steel axle tube on either side of the carrier housing:
- Traditional BOM numbers: six digits plus a dash and one or two digits (e.g., 603994-1)
- Numbers starting with 60 or 61 are traditional format
- Some tags omit the leading 60 — a stamp reading 3994-1 needs the 60 added back for database searches
- Modern Dana axles use a ten-digit format starting with 200 — no digits are omitted here
Running the full BOM number through Dana’s parts database returns the ring gear diameter, spline count, gear ratio, and carrier type.
Electronic Locking Differential Speed Limits
If your Ford rear axle codes show an L-prefix code (F-150) or a code ending in E (Super Duty), you’ve got an electronic locker. Ford’s owner manual content specifies the exact speed thresholds for locker engagement:
Normal, Sport, and Weather drive modes:
- Engages up to 20 mph (30 km/h)
- Auto-disengages above 25 mph (41 km/h)
- Re-engages when speed drops back below 20 mph
Off-road modes (Mud/Sand, Baja, Rock Crawl):
- No automatic disengagement — the locker stays locked at any speed
4L (Four-Wheel Low):
- Engages up to 55 mph (90 km/h)
- Auto-disengages above 62 mph (100 km/h)
Knowing these limits matters when you’re wheeling in 4L and suddenly wonder why the locker is still engaged at highway speed — in 4L, that’s by design.
Known Failure Points Worth Knowing
Sterling 10.25-inch (1985–1992): Early short pinion-yoke design allowed the yoke nut to work loose under heavy towing, leading to gear misalignment and bearing failure. Ford redesigned this with a longer pinion yoke in 1993, improving thread engagement significantly.
Sterling 10.5-inch (1999 onward): The axle tubes are pressed into the cast iron center section and secured with plug welds. Under extreme torque loads — especially modified diesels — those plug welds can shear, letting the tube rotate inside the housing. The result is spring pad misalignment, driveshaft failure, and loss of vehicle control. Ford’s fix for high-torque applications was transitioning to the Dana M275 and M300, which use thicker tubes and more robust housings.
If your axle code points to a Sterling 10.5 and you’re running a modified diesel or pulling heavy loads regularly, that failure mode is worth monitoring.
