MGM ✓ Factory-verified

GMC · Regular Production Option

Transmission, Allison 10 speed automatic

Years 2020–27 Models 1 Factory citations 3

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Also documented for Chevrolet

Documented applications

Sierra HD — T1XX (2020–2027) →

• 2020–2027 ✓ Factory-verified

  with the Duramax diesel; the 2026 guides print it without Allison; 2020 to 2024: GM Authority Sierra HD model-year pages pair MGM with the Duramax diesel

  ⛭ GM factory document — 2025 gmc sierra 2500 hd retail and fleet fleet order guide rpo codes, p.1.

  ⛭ GM factory document — 2026 gmc sierra 2500 hd retail and fleet fleet order guide rpo codes, p.1.

  ⛭ GM factory document — 2027 gmc sierra 2500 hd retail and fleet fleet order guide rpo codes, p.1.

This page aggregates every documented appearance of MGM in the GMC reference. Confidence and citations are shown per application — the same code can be better documented in one year than another.