GU5 is the RPO code for a 3.23 rear axle ratio — the tallest common gear GM puts in a full-size truck. If it is on your label, your truck was ordered for road miles, not for pulling. The GM rear axle RPO code chart lists it alongside every other GM ratio.
It is also the ratio that shows up most often in fleet and police specifications, which is why so many of our records for it come from those documents rather than from retail order guides.
Where GU5 is documented
| Year | Platform | Source |
|---|---|---|
| 1988 | — | GM option master (“AXLE REAR, 3.23 RATIO”) |
| 2007 | GMT800 | 2007 Chevrolet Silverado Classic Truck Restoration Kit |
| 2021, 2023, 2024 | T1XX | GM Upfitter police and special-service specification guides (Tahoe PPV, Silverado SSV, Suburban municipal) |
| 2022 | T1XX | GM Upfitter guides, plus a secondary transcription we do not treat as primary |
| 2025–2026 | T1XX | GM Fleet order guides; 2026 Tahoe 9C1 specification guide; GM Canada eOGLD |
Twenty-six GM-sourced records, one meaning throughout. We hold no GM-sourced record for GU5 between 1989 and 2006, or for GMT900 and K2XX — only an unverified 2019 entry that did not meet our bar. Those are gaps in our library, not gaps in GM’s catalog.
What 3.23 is good at, and what it is not
A tall ratio keeps engine speed down at cruise. On a long highway commute that is worth real fuel, and it is quieter. Put a trailer behind it and the same ratio becomes the problem: less torque multiplication means the transmission has to hold a lower gear longer, which puts heat into the transmission rather than into the axle.
That is why GU5 trucks so rarely carry NHT, and why a 3.23 truck advertised as a tow rig is worth a second look at the label.
| GU5 — 3.23 | GU6 — 3.42 | GT4 — 3.73 | |
|---|---|---|---|
| Empty-truck economy | Best | Middle | Worst |
| Towing | Weakest | Adequate | Best |
| Tolerance for bigger tires | Lowest | Middle | Highest |
GU5 is the ratio that punishes a tire upsize
This is the practical reason to care. Going from a 31-inch to a 33-inch tire on a 3.23 axle lands your effective gearing near 3.03. There is no factory ratio that tall. The truck will feel noticeably slower, the transmission will shift more, and towing anything meaningful becomes unpleasant.
If you have GU5 and want larger tires, regearing is not an upgrade — it is the thing that makes the tires liveable. Start with GMT800 regearing for bigger tires or how to regear GMT400 axles for the older platforms.
What GU5 does not tell you
- Whether the differential locks. A 3.23 can be open or G80-equipped.
- Whether the truck is two- or four-wheel drive. GU5 describes the rear ratio only; on a 4×4 the front carries the same ratio by necessity.
- What the axle holds today. The label is a record of the build, not an inspection.
How to confirm it
- Decode GU5 for your model year.
- Read the axle tag — a 42/13 ring-and-pinion count is 3.23.
- Count driveshaft turns against two full rear-wheel revolutions. Close to 3.23 confirms it.
Sources
- 2007 Chevrolet Silverado Classic Truck Restoration Kit, GM Heritage Archive
- 2020–2024 GM Upfitter police and special-service package specification guides
- 2026 Tahoe 9C1 Police Package Specification Guide, GM Fleet
- 2025–2026 GM Fleet order guides; GM Canada eOGLD order guide
- GM VPPS option master, November 2002
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