G80 is the RPO code for GM’s locking rear differential — and it is the most looked-up code on our decoder by a wide margin. It is also the code most likely to mislead you, because G80 has not meant the same piece of hardware for the last fifty years. On a 1973 C10 it is a Positraction clutch-pack. On a 2026 Silverado it is an automatic locker. Same three characters, different axle. Every GM axle ratio and differential code is side by side in our GM rear axle RPO code chart.
What G80 means, by generation
These are the years we hold a GM document for, in GM’s own wording. They are not every year G80 was offered — they are every year we can prove it.
| Generation | Year documented | How GM described it | Source |
|---|---|---|---|
| Squarebody | 1973 | “Positraction rear axle” | 1973 Chevrolet Truck Vehicle Information Kit |
| GMT400 | 1990 | “Locking differential” | 1990 Chevrolet Truck Vehicle Information Kit |
| — | 1991 | “Axle Positraction, limited slip” | GM option master |
| GMT400 / GMT800 | 2000 | “Rear limited-slip / Positraction axle” | 2000 Chevrolet C/K & Silverado Truck Restoration Kit |
| GMT800 | 2007 Classic | “Limited-slip / Positraction rear axle” | 2007 Chevrolet Silverado Classic Truck Restoration Kit |
| K2XX | 2016 | “Locking differential” | 2016 Special Applications: Snowplow Prep Package |
| T1XX | 2021–2026 | “Heavy-duty locking”, then “Auto-locking rear differential” from 2022 onward | GM Fleet order guides; 2026 Silverado 9C1 spec guide |
GMT900 (2007–2013) is a gap. We do not hold a GM document that lists G80 for those years. It was certainly offered; we just will not print a year we cannot source.
Positraction and a locker are not the same thing
This is the part that costs people money.
| Limited-slip / Positraction | Automatic locker | |
|---|---|---|
| What it does | Always sends some torque to both wheels through clutches or gears | Runs open, then mechanically locks both axle shafts together when one wheel spins |
| How it feels | Progressive. You rarely notice it engage | A distinct clunk or bang when it locks, usually at low speed |
| Service | Friction modifier in the gear oil matters | Internal mechanism; fluid spec still matters, modifier requirement differs |
| Rebuildable at home | Often | Usually not worth it |
GM’s own wording moved from “Positraction” and “limited slip” in the early documents to “locking” and finally “auto-locking” in the current order guides. That wording change tracks a real change in the hardware. What our records hold is the wording — the exact unit in your axle is settled by the axle tag and the parts catalog for your build, not by the RPO alone.
How to tell what you actually have
- Find the RPO label — glove box lid, console lid, or under the spare.
- Decode G80 against your model year, not against a generic list.
- Jack up the rear with both wheels off the ground, in neutral, and turn one wheel by hand. If the other turns the same direction, you have some form of limited-slip or locker. If it turns the opposite direction, the axle is open — whatever a listing said.
- Read the axle tag for the ratio and build code. The ratio lives in a separate RPO: GU5 3.23, GU6 3.42, GT4 3.73.
What G80 does not tell you
- It is not a ratio. G80 says the differential locks. It says nothing about gearing. A 3.23 G80 and a 3.73 G80 are two very different trucks to drive.
- It is not a tow package. That is Z82, or NHT for max tow.
- It is not an off-road package. That is Z71, which on many trucks includes G80 but is not the same code.
- It does not survive an axle swap. The label records what left the factory. If the rear end has been changed, the label is now a history note.
If your label is a QR code
On 2018 and newer trucks the glove box label often carries a QR code instead of the printed list. Scan it with your phone camera and paste the result into a notes app — phone cameras usually truncate it on screen.
It is not a web link. It is one long comma-separated line: VIN, model year, model, build date, order number, then a group of three-character codes separated by spaces, then the paint code. Paste that group straight into the RPO decoder.
Sources
- 1973 and 1990 Chevrolet Truck Vehicle Information Kits, GM Heritage Archive
- 2000 Chevrolet C/K & Silverado Truck Restoration Kit, GM Heritage Archive
- 2007 Chevrolet Silverado Classic Truck Restoration Kit, GM Heritage Archive
- 2016 Special Applications – Full Size K Series (4×4): Snowplow Prep Package, GM Upfitter
- 2025–2026 GM Fleet order guides; 2026 Silverado 9C1 Police Package Specification Guide
- GM VPPS option master, November 2002
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