GM Rear Axle RPO Codes – Gear Ratios (Chevy / GMC Trucks)

If you’re trying to figure out what rear end is in your Chevy or GMC truck, your RPO codes tell the story.

These codes are found on the glovebox sticker (older trucks), inside the center console, or in the service parts identification label depending on year. They define your factory gear ratio and whether you have a locking differential.

Here’s the breakdown for the most common full-size GM truck rear axle codes:

GM Rear Axle RPO Code Decoder

Type your code, your ratio, or what you are trying to do. Squarebody through T1XX. Every row is marked with how well GM documentation actually backs it.

📌 Standard & Common Gear Ratios

GQ1 — Standard Ratio (unconfirmed)

• Widely reported as the base rear axle ratio for a given model and year, varying by configuration

• We could not confirm GQ1 in GM order guides, GM Body Builder Manuals or driveline industry code listings. Treat it as owner-reported until you can match it against a build sheet — and if you have documentation for it, post it on the forum and we will correct this page

GU4 — 3.08 Ratio

• Lower RPM at highway speeds

• Best for fuel economy

• Not ideal for heavy towing

GU5 — 3.23 Ratio

• Mild balance between mileage and light-duty towing

GU6 — 3.42 Ratio

• Very common in half-ton trucks

• Strong all-around ratio

• Good towing and drivability balance

GT4 — 3.73 Ratio

• Popular towing gear

• Better low-end pull

• Slightly higher cruising RPM

GT5 — 4.10 Ratio

• Strong acceleration

• Better for heavier trailers

• Higher highway RPM

HC4 — 4.56 Ratio

• Rare factory option

• Heavy towing / off-road focus

• High cruising RPM

🔒 G80 — Locking Rear Differential

G80 — automatic locking rear differential

This is GM’s factory automatic locking rear differential, common in 1500, 2500 and 3500 trucks. Two things trip people up here. First, GM’s own paperwork has never settled on one description. Our GM RPO Codes Database holds eight separate GM-sourced entries for G80, and the wording flips back and forth across fifty-three model years: “Positraction rear axle” in 1973, “locking rear differential” in 1990, “P, AXLE POSITRACTION, LIMITED SLIP” in 1991, “rear limited-slip / Positraction axle” in 2000, “limited-slip / Positraction rear axle” on the 2007 Silverado Classic, back to “locking rear differential” for 2016 4x4s, then “auto-locking rear differential” for 2022–2023 and “automatic locking rear differential” for 2026. So the Positraction wording is not a typo GM forgot to clean up — it is what several of their own documents actually say. Mechanically it is still a locker rather than a clutch-type limited slip, whatever a given year’s paperwork calls it. Second, the hardware behind the code changed over a very long production run. On older trucks it is the Eaton governor-actuated automatic locker everyone calls the “Gov-Lock” (Eaton’s current mechanical automatic locker is sold as the MLocker), so do not assume a late-model G80 is the same unit as a 1990s one.

Two more model-specific catches: on 2019-and-newer T1XX trucks, G80 sits alongside the driver-selectable electronic lockers G94 (rear) and G93 (front) — different codes, different behaviour. And on 2021-and-newer Tahoe, Suburban, Yukon and Escalade, the differential option code is G86 (limited slip), not G80 at all. If you are decoding an SUV sticker and cannot find G80, that is why.

How it works:

• Acts like an open diff in normal driving

• Automatically locks both axles together when wheel slip is detected

• Not a true clutch-style posi — it’s a mechanical locking system

Good for:

• Snow

• Mud

• Light off-road

• Work trucks

Note:
If you see G80, that means you have a locking differential regardless of gear ratio.
You will also have one of the ratio codes listed above.

🛠 Common Gear Ratio Pairings (What You’ll Usually See)

🔎 Where to Find Your RPO Codes

• 1988–2006: Glovebox sticker

• 2007+ Classic: Glovebox

• 2007+ New Body & newer: Check center console or VIN build sheet

If your glovebox sticker is gone, you can:

• Decode by VIN through GM build sheet services

• Physically verify by spinning the driveshaft and counting rotations

• Pull the diff cover and confirm ring gear stamping

If you’re swapping gears, running larger tires, or trying to match front/rear ratios in a 4×4, confirm your exact RPO before ordering parts.

Got the codes off the label? Look up any GM axle RPO code in the GM RPO Codes Database, or put the whole list into the GM RPO Decoder and it will tell you what the truck was built with for its model year.

🔧 Physically Identifying Your GM Rear Axle

The RPO code sticker is the fastest way to identify a stock axle, but stickers fade, glove boxes get replaced, and trucks change owners. Three physical checks confirm what you actually have:

• Axle tube tag: Look for a small metal tag bolted to one of the rear differential cover bolts (typical on 10-bolt and 14-bolt corporate axles). The tag stamping lists ratio and date code.

• Ring gear stamp: Pull the diff cover, drain the gear oil, and inspect the ring gear face. The ratio is etched into the ring gear (for example, 3.42 or 41-12 where 41/12 = 3.42).

• Tooth count: If the stamp is unreadable, count the teeth on the ring gear and divide by the pinion tooth count. 41 ring / 12 pinion = 3.42:1.

🛞 Gear Ratio vs Tire Size — How Larger Tires Change Your Effective Ratio

Running a bigger tire than stock effectively tells your axle it is geared lower than it actually is. Use this rough math: effective ratio = factory ratio × (new tire diameter ÷ stock tire diameter).

For example, a GMT800 with a stock 3.73 ratio and stock 31.6-inch tires (P265/70R17) running 35-inch tires has an effective ratio of 3.73 × (31.6 ÷ 35) = ~3.37. That is the same as dropping from 3.73 to roughly 3.42 gears — meaningful loss of off-the-line pull and worse on a tow vehicle.

Common targets when sizing up tires:

• Stock 31s with 3.42 gear → upgrade to 33s, consider re-gearing to 3.73.

• Stock 32s with 3.73 gear → upgrade to 35s, consider re-gearing to 4.10.

• Stock 33s with 3.73 gear → upgrade to 37s, plan on 4.56.

Verify with the actual tire diameter, not the size code on the sidewall — measured circumference can differ from nominal by an inch or more depending on manufacturer and load.

🪛 Common Axle Housing Identifiers

The RPO code tells you the ratio; the axle housing tells you which physical axle you have. For most full-size GM trucks:

• 10-bolt (8.5″ or 8.6″): Standard half-ton rear (Silverado/Sierra 1500). Strong enough for stock-power daily use.

• 14-bolt semi-float (9.5″): 1500HD and older 2500 light-duty applications. Stronger than the 10-bolt; common upgrade target.

• 14-bolt full-float (10.5″): 2500 HD and 3500 trucks. The heavy-duty unit. Identifiable by the protruding center hub on each axle end.

• 9.25″ IFS front (4WD): Front independent axle on most GMT400/GMT800/GMT900. RPO codes for the front match the rear ratio code in 4WD trucks.

Sourcing a donor axle or planning a swap? The GM full-size truck parts interchange master list covers which axles, brakes and gearing cross over between Squarebody, OBS, GMT800 and GMT900 trucks.

❓ GM Rear Axle FAQ

How do I know if I have a G80 locker?
Check the glove box RPO sticker for the G80 code. Physically, with the truck in neutral and the rear wheels off the ground, spin one rear wheel by hand — on an open differential the other wheel spins the opposite direction; on a G80 both wheels tend to lock together once a slip threshold is exceeded.

Can I swap gear ratios without changing the axle?
Yes. A gear swap requires a ring-and-pinion kit, new bearings, a setup pattern check, and proper pinion preload. Plan on $400–$800 in parts plus shop labor if you don’t have setup tools. Always replace ratios in matched pairs front and rear on 4WD.

Will changing tire size hurt my transmission shift points?
Yes, on any electronically controlled transmission. Larger tires raise effective gearing and delay shifts. A speedometer recalibration and a tune to correct shift schedules are recommended whenever you size up by more than one inch of overall diameter.

What RPO ratio is best for towing under 7,000 lb?
3.73 is the most common towing ratio in stock GMT800/GMT900/K2XX trucks. 3.42 is acceptable for light loads. 4.10+ is the right choice for heavy or frequent towing combined with larger tires.

Why does my truck have a G80 RPO but no locking action?
The G80 Gov-Lock uses a flyweight mechanism that wears out. High-mileage trucks (200K+) often have failed Gov-Locks that function as open differentials. A rebuild kit or swap to a true selectable locker (Eaton ELocker, ARB Air Locker) is the long-term fix.

💬 Share Your Axle Setup on the FSC Forum

Running a non-stock ratio, a swapped axle, or a Gov-Lock failure story? Post your setup, your symptoms, and what you ended up doing in the FSC GMT800 / GMT400 / GMT900 community threads.

👉 Join the discussion on the FSC Forum