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Monster TH400
The legendary 3-speed heavy-duty transmission. Perfect for classic big-block or LS-powered 2500s where strength and simplicity are key.
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GMC Sierra 2500 Transmissions
From classic TH400 heavy-duty builds to modern Allison-equipped Duramax trucks, Monster Transmission offers Sierra 2500 solutions built for towing, hauling, and reliability. Work, fleet, or performance—we’ve got you covered.
Engineered For Sierra 2500 HD Demands
Monster Transmissions are purpose-built for GMC Sierra 2500 owners who need more than stock. With upgraded internals, increased clutch capacity, and dyno-tested confidence, our builds withstand the torque of big-blocks, LS swaps, and Duramax diesels.
From vintage TH400 swaps to modern Allison 1000 workhorses, we match the build and torque converter to your exact application.
Monster 4L80E
4-speed overdrive built to handle towing and big torque. Monster upgrades address direct clutches and planetary strength.
Shop 4L80EMonster 4L85E
Heavy-duty evolution of the 4L80E with higher torque capacity. Great for hauling, plowing, or performance builds.
Shop 4L85EMonster 6L90E
Modern 6-speed with added clutch capacity and gear spread for towing and drivability. Our builds solve 4-5-6 clutch and valve-body issues.
Shop 6L90EMonster Allison 1000
The gold standard for Duramax-equipped Sierra HDs. Monster Allison builds deliver superior heat management and torque handling.
Shop Allison 1000Which Transmission Do I Have? — Sierra 2500 / 2500HD
Typical Sierra 2500/2500HD pairings by engine and model year. Edge cases exist—always verify by VIN or transmission tag.
| Transmission | Engine(s) | Typical Model Years | Notes |
|---|---|---|---|
| TH400 | Big-block gas (HD) | ~1973–1990 | 3-speed heavy-duty; common in pre-OD HD use |
| 4L80E | 5.7L / 7.4L gas; early HD | ~1991–1998 | Electronic 4-speed OD; HD workhorse of the ’90s |
| 4L85E | LS-based V8 (select HD) | ~1999–2007 | Higher-capacity 4L80E; 2007 Classic at end of run |
| 6L90E | 6.0L / 6.2L gas (HD) | ~2007–2021 | Modern 6-speed for gas HD; improved gear spread |
| Allison 1000 | 6.6L Duramax diesel | ~2001–2019 | Duramax-only factory pairing on 2500/3500 HD |
Why Sierra 2500 Owners Choose Monster
From dyno-tested builds to diesel-specific solutions, Monster ensures your 2500 is ready for work and play.
Heavy-Duty Components
Corrects GM weak points with billet shafts, stronger clutches, and calibrated hydraulics.
Matched Torque Converters
Spec’d to your power, gears, tires, and use case for lockup durability and heat control.
Dyno-Tested
Every 2500 transmission is live-tested to verify shift quality and performance before shipping.
Diesel & Gas Expertise
Solutions tailored for Duramax diesels and gas big-blocks alike.
Warranty Options
Baseline coverage with extended options for heavy commercial or towing applications.
Nationwide Shipping
Fast delivery on popular 2500 units with core return support.
Talk to an Expert
Need help choosing the right Sierra 2500 transmission?
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GMC Sierra 2500 Transmission FAQ
Answers to common GMC Sierra 2500 HD transmission questions.
Which transmission does my Sierra 2500 use?
Depends on year/engine. Older 2500s often ran TH400 or 4L80E; HD variants add 4L85E and later 6L90E. Duramax diesels are typically paired with the Allison 1000.
Do Monster builds fix common 4L80E/6L90E issues?
Yes—our packages target clutch durability, valve-body wear, and heat management for towing and high-load use.
Is the Allison 1000 better than the 6L90E?
For Duramax diesels, the Allison is optimized for high torque. For gas engines, the 6L90E is common. We strengthen both for reliability and longevity.
Do your transmissions include a torque converter?
Yes. Every Sierra 2500 Monster build includes a converter matched to your engine, axle ratio, tire size, and usage.
How soon can I get my Sierra 2500 transmission?
Popular units like 4L80E, 6L90E, and Allison 1000 are often in stock. Check the product page or call for an accurate ETA by build level.