Chevy Axle Ratio by VIN: How to Find the Factory Gear Ratio
You can find a Chevrolet's factory axle ratio by VIN, but not by reading the VIN itself. None of the 17 characters stands for the gear ratio, so the ratio has to come from the vehicle's factory build record, where Chevrolet lists the rear axle as an option code such as GU6 (3.42), GT4 (3.73) or GT5 (4.10) on many trucks, and a locking differential as G80. Enter the VIN in the VIN lookup: when a record is available you get the original window sticker or, if there is no sticker, an RPO build sheet with the axle codes and their descriptions. That record shows the ratio the vehicle was built with. Whether the axle under it today is still original is a separate question, and the only way to answer it is a physical check.
What the VIN characters can and cannot tell you
A standard VIN decode identifies the manufacturer, model, model year, assembly plant and a handful of encoded attributes. The axle is not one of them.
| Detail | From a standard VIN decode | Needs the factory record |
|---|---|---|
| Model, model year, plant | Yes | — |
| Engine | Yes, via the 8th character | The exact engine code |
| Drive layout (2WD, 4WD, AWD) | Often, depending on model | Transfer case details |
| Axle ratio | No | Yes |
| Locking or limited-slip differential | No | Yes |
| Trailering equipment | No | Yes |
This is why two Silverados whose VINs decode to the same model year, cab, engine and drive can still carry different gearing. The difference lives in the option content, not in the VIN characters. For what each VIN position does encode, see how to read a Chevrolet VIN.
Reading axle codes in the build record
On many GM trucks and SUVs, the ratio and the differential type are separate codes:
| Code | Typical meaning on many GM trucks |
|---|---|
| GU6 | 3.42 rear axle ratio |
| GT4 | 3.73 rear axle ratio |
| GT5 | 4.10 rear axle ratio |
| G80 | Automatic locking rear differential (not a ratio) |
Code meanings vary by model and year, so always read the description printed beside the code rather than relying on the table alone. A few points help when reading the list:
- One ratio code covers the vehicle. On four-wheel-drive trucks the front differential is geared to match the rear, so the record normally shows a single ratio.
- G80 rides alongside a ratio code. A truck can have 3.42 gears with or without the locker, and the record shows both facts separately.
- Front-wheel-drive models work differently. On cars and crossovers with a transaxle, the equivalent is the final-drive ratio inside the transaxle, and factory records describe it that way.
- Codes outside the common set can be checked in the RPO code navigator.
On a window sticker, the axle may appear as a named line item, sometimes with no separate price when it came as part of a package or was standard for that configuration.
Other places the factory ratio is recorded
If the vehicle is in front of you, the same codes may be printed on the Service Parts Identification label that GM placed in many vehicles, usually in the glove box; reading the SPID label covers how to find and decode it. Many GM axles also carry a stamped build code or a tag on the housing or cover, which a driveline shop can interpret. Labels go missing and covers get swapped, so treat these as supporting evidence next to the build record.
Confirming the axle still matches the record
The build record tells you what was installed at the plant. Regearing is common on trucks fitted with larger tires, and axle assemblies get replaced after damage, so a physical check is worth doing before you rely on the number:
- With the vehicle safely supported and both rear wheels off the ground, put the transmission in neutral.
- Mark the driveshaft and one tire.
- Turn both rear wheels together through one full revolution and count the driveshaft turns. A count of about 3.4 points to 3.42 gears, about 3.7 to 3.73, and so on.
If only one wheel turns while the other stays still on an open differential, the count is roughly half the ratio. A shop can also count ring and pinion teeth with the cover off. If the measured ratio disagrees with the record, ask whether the speedometer and transmission calibration were updated after the change, because a regear or tire-size change throws both off.
Why the factory ratio matters
A numerically higher ratio pulls harder from a stop and suits heavy loads, while a lower ratio keeps engine speed down on the highway. Chevrolet's published trailer and combined weight ratings for a given truck are tied to its axle ratio along with engine, cab and drive, so the ratio is one input to towing capacity rather than the whole answer. The rest of the trailering equipment, from hitch to brake controller, is covered in Chevy towing package by VIN.