GU6 is the RPO code for a 3.42 rear axle ratio. On modern half-tons it is the single most common ratio GM builds, which makes it the code most owners find on their label and then misread as evidence of something special. It is not. It is the default. See where it sits against every other GM ratio in the GM rear axle RPO code chart.
Where GU6 does matter is on the used market, because it is the ratio GM pairs with the NHT Max Trailering Package. Seeing 3.42 on a listing is not proof of max tow — but a max-tow truck without it would be unusual.
Where GU6 is documented
| Year | Platform | Source |
|---|---|---|
| 1983 | — | GM option master (“AXLE REAR, 3.42 RATIO”) |
| 2000 | GMT400 / GMT800 transition | 2000 Chevrolet C/K & Silverado Truck Restoration Kit |
| 2007 | GMT800 | 2007 Chevrolet Silverado Classic Truck Restoration Kit |
| 2021–2023 | T1XX | GM Upfitter special-service and police specification guides |
| 2022 | T1XX | Also in a secondary order-guide transcription we do not treat as primary |
| 2025–2026 | T1XX | GM Fleet order guides; GM Canada eOGLD |
Forty-three years of documents and the meaning never moves: 3.42, every time. The gaps in the middle — GMT900 and K2XX — are years we hold no GM-sourced record for, only an unverified 2019 entry. They are not years GU6 went away.
Why 3.42 became the default
It is the ratio that does not commit. Low enough that an empty truck is not screaming at highway speed, high enough that the truck will still pull a reasonable trailer without the transmission hunting. Once GM moved to eight- and ten-speed automatics, the extra gears did the work that a taller or shorter axle used to do, and 3.42 became the one ratio that suited nearly every configuration.
| GU5 — 3.23 | GU6 — 3.42 | GT4 — 3.73 | |
|---|---|---|---|
| Typical use | Fleet and highway | The standard half-ton | Tow and work |
| Relative cruise rpm | −6% vs 3.42 | Baseline | +9% vs 3.42 |
| Paired with max tow | No | Yes — NHT lists 3.42 | Sometimes |
The mistake GU6 causes
Owners upsize tires on a 3.42 truck and expect it to feel the same. A 33-inch tire on a stock-height half-ton drops your effective ratio to roughly 3.15, which is below the lowest ratio GM offered. The truck gets lazy off the line, the transmission downshifts more, and fuel economy usually gets worse rather than better.
If you are planning tires, read GMT800 regearing for bigger tires first — the arithmetic is the same on any platform.
What GU6 does not tell you
- It is not NHT. NHT lists 3.42 in GM’s documents, but 3.42 on its own is just the standard ratio.
- It is not a locking differential. That is G80.
- It does not describe the axle size. A 3.42 can sit in a small or a large ring gear. The axle code and the NHT 9.76-inch listing are what speak to that.
How to confirm it
- Decode GU6 for your model year.
- Look for the ring-and-pinion tooth count on the axle tag — 41/12 is 3.42.
- Or count it: two full turns of a raised rear wheel should give you close to 3.42 driveshaft revolutions. This is the only check that reflects the axle as it is now.
Sources
- 2000 Chevrolet C/K & Silverado Truck Restoration Kit, GM Heritage Archive
- 2007 Chevrolet Silverado Classic Truck Restoration Kit, GM Heritage Archive
- 2021–2023 GM Upfitter special-service and police package specification guides
- 2025–2026 GM Fleet order guides; GM Canada eOGLD order guide
- GM VPPS option master, November 2002
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