In GM heavy-duty pickups, the transmission depends on the engine. The 6L90 is the heavy-duty six-speed behind the gas V8s in 2500 and 3500 trucks and in vans, while the Allison 1000 was paired with the Duramax diesel for many years. The 6L90 shares its gear ratios with the lighter 6L80 and is a capable gas-truck transmission; the Allison is built for diesel torque and heavy towing. They are not interchangeable between the two engine families.
6L90 vs Allison 1000 at a glance
| 6L90 | Allison 1000 | |
|---|---|---|
| Engines | Gas V8s in heavy-duty trucks and vans | Duramax diesel, and some heavy-duty gas engines |
| Gears | 6 | 5 in early years, then 6 |
| Gear ratios | 4.03 / 2.36 / 1.53 / 1.15 / 0.85 / 0.67 | 3.10 / 1.81 / 1.41 / 1.00 / 0.71 (6-speed adds 0.61) |
| Fluid | Dexron VI | Allison-approved fluid or the Dexron GM specifies |
Do all Duramax trucks have an Allison?
For many years, yes: the Allison 1000 was the automatic behind the Duramax in GM heavy-duty pickups. Later trucks moved to a ten-speed automatic, so check the build data for the exact transmission in a given truck.
How they differ in use
The 6L90’s deep first gear helps a heavy gas truck launch, and its two overdrives keep highway rpm down. The Allison’s gearing suits a diesel’s broad torque, and it is widely regarded as durable under heavy towing. Both last longest with clean fluid and a healthy cooler.
Towing with a gas heavy-duty truck
A gas heavy-duty truck with a 6L90 works harder on long grades than a diesel, revving higher and downshifting more often, which raises fluid temperature. Use tow/haul mode, watch the transmission temperature if your truck shows it, and shorten the fluid service interval if you tow often; see tow/haul mode and transmission overheating.
Common problems
- 6L90: similar issues to the 6L80, such as converter shudder and pump wear; see 6L80 vs 6L90 and 6L80E problems.
- Allison 1000: shift quality issues, converter and clutch wear with added power, and overheating under extreme loads; see Allison 1000 problems.
To confirm your code, check the build data or the identification tag on the transmission; see what transmission do I have?
Replacement units we list
- RoadReady Chevy 6L90E: $2,599.00
- StreetMonster Chevy 6L90E: $3,199.00
- SportMonster Chevy 6L90E: $3,899.00
- SportMonster Chevy Allison 1000: $5,625.00
Each includes a torque converter. Browse 6L90E transmissions and Allison 1000 transmissions, and compare with the Ram option in 68RFE vs Allison 1000.
More guides: how long an Allison 1000 lasts · how long a 6L90 lasts.
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