GMC Sierra HD Five Generations (2001-present)

Five generations spanning 2001-present, sister truck to Chevrolet Silverado HD. Built at GM Flint Assembly, Michigan throughout. 1st gen GMT800 (2001-2007): launched 2001 as the inaugural Sierra HD line replacing C/K3500. 6.0L LQ4 Vortec gas V8 (300 hp / 360 lb-ft) base. 8.1L Vortec L18 gas V8 (340 hp / 455 lb-ft, big-block, discontinued mid-2007). 6.6L LB7 Duramax V8 turbo-diesel (235 hp / 500 lb-ft, 2001-2004) — Isuzu / GM joint development, first-generation Duramax. 6.6L LLY Duramax (310 hp / 605 lb-ft, 2004.5-2006). 6.6L LBZ Duramax (360 hp / 650 lb-ft, 2006-2007). Allison 1000 5-speed (early) / 6-speed (later) automatic. 2nd gen GMT900 (2007-2014): redesigned for 2007 (often called 2007.5+). 6.0L Vortec gas (353 hp 2007-2009, 360 hp 2010+). 6.6L LMM Duramax (365 hp / 660 lb-ft, 2007-2010). 6.6L LML Duramax (397 hp / 765 lb-ft, 2011-2014). 6-speed Allison automatic. 3rd gen K2HD (2015-2019): redesigned for 2015 with mostly carryover powertrains. 6.0L Vortec gas V8 (360 hp). 6.6L LML Duramax continued (397 hp 2015-2016). 6.6L L5P Duramax introduced 2017 (445 hp / 910 lb-ft — completely redesigned engine block, all-new GM-developed control system). 6-speed Allison automatic. 4th gen T1HD (2020-2023): completely new architecture and styling. 6.6L L8T gas V8 (401 hp / 464 lb-ft, replacing 6.0L Vortec — larger displacement, FlexFuel-capable). 6.6L L5P Duramax V8 turbo-diesel (445 hp / 910 lb-ft). Allison 10-speed automatic introduced (replacing 6-speed — major upgrade). 2022 mid-cycle refresh updated styling, added new front fascia. AT4X added 2024 — premium off-road trim with Multimatic DSSV spool-valve dampers (also used in Yukon AT4 / Hummer EV), electronic locking front + rear differentials, factory-installed AEV-developed wheels and tires. 2024+ T1HD refresh / 5th gen (often called comprehensive refresh of T1HD). Extensively restyled grille / fascia / interior. 6.6L L8T gas V8 (401 hp / 464 lb-ft, with 10-speed gas now towing 26,000 lb max — 2,000 lb improvement). 6.6L L5P Duramax uprated to 470 hp / 975 lb-ft from 2024 model year (25% more torque at lower RPM for improved climbing and trailering). Allison 10-speed standard. Towing up to 22,500 lb 2500HD / 36,000 lb gooseneck 3500HD DRW. Trims: Pro, SLE, SLT, AT4 (off-road), Denali (luxury), AT4X (premium off-road, 2024+), Denali Ultimate (luxury flagship, 2025+ — Vader Chrome grille, exclusive Alpine Umber interior with full-grain leather + open-pore Paldao wood + 16-way power massage seats, 12-speaker Bose Centerpoint, illuminated power assist steps, MultiPro Tailgate Audio System by Kicker). MultiPro Tailgate (six-function: primary gate, primary gate with load stop, full-width step, full-width step with load stop, easy-access mode, inner gate load stop) standard SLT+. Up to 14-camera views with trailer-camera integration. Up to 13.4-inch Google built-in infotainment + 12.3-inch digital cluster + multi-color head-up display. Super Cruise hands-free driving available with trailer-tow capability. Payload up to 7,290 lb (3500HD DRW).