GT4 is the RPO code for a 3.73 rear axle ratio. It is the working gear — the one GM put under trucks meant to pull something, and the one that makes a half-ton feel like it has an engine it does not actually have.
Unlike most RPO codes, GT4 means exactly one thing. Every record we hold, from 2000 to 2026, says 3.73. There is no year where it means something else. To compare it with every other GM ratio and the locker codes, see the GM rear axle RPO code chart.
Where GT4 is documented
| Year | Platform | Source |
|---|---|---|
| Undated | — | GM option master (“AXLE REAR, 3.73 RATIO”) |
| 2000 | GMT400 / GMT800 transition | 2000 Chevrolet C/K & Silverado Truck Restoration Kit |
| 2007 | GMT800 | 2007 Chevrolet Silverado Classic Truck Restoration Kit |
| 2016 | K2XX | 2016 Special Applications: Snowplow Prep Package |
| 2022 | T1XX | Secondary order-guide transcription only. No GM-hosted document for this year in our records |
| 2025–2026 | T1XX | GM Fleet order guides; GM Canada eOGLD |
Every one of the nineteen records we hold for GT4 is GM-sourced. It is one of the cleanest codes in the database — no conflicting meanings, no unverified rows.
What 3.73 actually does to your truck
A numerically higher ratio turns the driveshaft more times per wheel revolution. That multiplies torque at the wheels and raises engine rpm at any given road speed.
| GU5 — 3.23 | GU6 — 3.42 | GT4 — 3.73 | |
|---|---|---|---|
| Built for | Highway economy | The middle ground | Pulling and hauling |
| Cruise rpm | Lowest | Middle | Highest — roughly 15% above 3.23 |
| Launch and grade | Softest | Adequate | Strongest |
| Fuel use unloaded | Best | Middle | Worst |
If your truck has GT4 and you are wondering why it drinks fuel empty, that is the ratio doing its job. It was not built to be efficient unloaded.
GT4 and bigger tires
This is the most useful thing about knowing you have 3.73. Larger tires effectively lower your gearing — a 33-inch tire on a 3.73 axle behaves roughly like a 3.42 on the stock size. Trucks that started with GT4 tolerate a tire upsize far better than a 3.23 truck does, which is why owners who plan to lift check this code before they buy wheels.
It also throws your speedometer off, and the correction is the same either way. We cover the arithmetic in GMT800 regearing for bigger tires and how to regear GMT400 axles.
What GT4 does not tell you
- Whether the differential locks. That is G80. A 3.73 open axle and a 3.73 G80 axle carry different codes for a reason.
- Whether the truck has a tow package. That is Z82, or NHT for max tow. GT4 often travels with them, but it is sold separately.
- What is in the axle now. The label records the build. Regeared trucks keep the original code.
How to confirm the ratio on your truck
- Decode GT4 for your model year and read the whole label while you are there.
- Check the axle tag or the differential cover stamping. On many GM axles the ring-and-pinion tooth count is stamped directly — 41/11 is 3.73.
- Count it. Mark the driveshaft and a rear tire, jack both rear wheels off the ground, and turn the tire exactly two full revolutions. Count driveshaft turns. Close to 3.73 confirms it, and this is the only method that reflects what is in the axle today rather than what left the factory.
Sources
- 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), GM Upfitter
- 2025–2026 GM Fleet order guides; GM Canada eOGLD order guide
- GM VPPS option master, November 2002
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