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

General Truck Discussion
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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:

:pushpin: Standard & Common Gear Ratios

GQ1 — Standard Ratio

  • Base rear axle ratio (varies by model/year)

  • Typically paired with base engine configurations

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

:locked: G80 — Locking Rear Differential

G80 — automatic locking rear differential (the Eaton “Gov-Lock” on older trucks)

Mechanically it is a locker, not a clutch-type posi. GM’s own paperwork does not say that consistently, though. The GM RPO Codes Database holds eight GM-sourced entries for G80, and the wording moves around across fifty-three model years:

Model year What GM’s document says
1973 Positraction rear axle (C/K pickup)
1990 locking rear differential (C/K and Sierra pickup)
1991 P, AXLE POSITRACTION, LIMITED SLIP
2000 rear limited-slip / Positraction axle (C/K and Silverado)
2007 limited-slip / Positraction rear axle (Silverado Classic)
2016 locking rear differential (Silverado and Sierra 4x4)
2022–2023 Auto-locking rear differential (Silverado 1500)
2026 automatic locking rear differential (Silverado 1500 9C1)

Same code, same basic behaviour, inconsistent description. So if you have seen G80 called Positraction somewhere and a locker somewhere else, both came from GM.

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

On newer trucks, check for the driver-selectable codes too. The database has G94 (“Differential, driver-selectable full-locking rear”) and G93 (front) as GM-sourced entries on the 2022–2023 Silverado 1500. Those lock when you press the button. G80 locks on its own. Different codes, different behaviour.

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.

Worth knowing: the Gov-Lock’s flyweight mechanism wears out. A 200K-mile truck can still carry G80 on its sticker and behave like an open diff. The sticker tells you what the truck left the factory with, not what it does today.

Quick test: rear wheels off the ground, truck in neutral, spin one rear wheel by hand. On an open differential the other wheel spins the opposite direction. On a working G80 the two tend to lock together once you exceed the slip threshold.

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

Use Case Recommended Ratio
Daily driver / highway 3.08 – 3.23
Balanced street + tow 3.42
Frequent towing 3.73
Heavy towing / larger tires 4.10+

:magnifying_glass_tilted_right: 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 the sticker is still readable, skip the hand-decoding: look up one code in the GM RPO Codes Database, or photograph the whole label and run it through the GM RPO Decoder to get the meanings that apply to your model year.

If you’re swapping gears, running larger tires, or trying to match front/rear ratios in a 4x4, confirm your exact RPO before ordering parts.