A lot of GMT400 owners eventually ask the same question:
“What gear ratio do I actually have?”
If you’ve got a K1500 or K2500 with a 700R4 / 4L60, there are a few quick ways to figure it out without pulling the differential apart.
1. Check the RPO Codes in the Glove Box
Open your glove box and look for the RPO sticker. This white sticker lists the factory option codes.
Common rear gear codes:
| Code | Gear Ratio |
|---|---|
| GU6 | 3.42 |
| GT4 | 3.73 |
| GT5 | 4.10 |
| HC4 | 4.56 (typically heavy duty / 8 lug) |
If you find one of these codes, that is the factory ratio the truck left the plant with.
Note: Previous owners may have swapped gears, so this only confirms original equipment.
2. Spin Test (Quick Garage Method)
If the RPO sticker is gone or unreadable:
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Put the rear axle on jack stands.
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Mark the driveshaft and one tire with chalk.
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Rotate the tire one full revolution.
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Count how many times the driveshaft rotates.
Approximate results:
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~3.4 turns = 3.42
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~3.7 turns = 3.73
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~4.1 turns = 4.10
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~4.5 turns = 4.56
If you have limited slip or both wheels spin together, rotate the tire once.
If it’s open differential, rotate the tire two full turns.
3. Common GMT400 Gear Ratios
Typical factory ratios found in these trucks:
K1500
-
3.42 (1988-1998)
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3.73 (1988-1998)
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4.10 (1988-1995)
K2500
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3.42 (1988-1999)
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3.73 (1988-1999)
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4.10 (1988-1999)
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4.56 (1989-1999 8 lug only)
4. Why It Matters
Your gear ratio affects:
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Towing capability
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Highway RPM
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Tire size compatibility
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Acceleration
Example:
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3.42 – better highway fuel economy
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3.73 – balanced daily driver
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4.10+ – better for towing or large tires
Thanks for reading!