2026 Automatic Gasoline The GMC Sierra 3500 HD truck is one of the most powerful heavy-duty trucks, specifically designed for strenuous towing and hauling tasks that exceed the capabilities of 1500 and 2500 trucks. The -2026 truck is available with two main engine options, both with a 6.6-liter capacity: 1. Gasoline Engine (6.6L V8) Power: 401 horsepower. Torque: 629 Nm (464 lb-ft). Transmission: 10-speed Allison automatic. Maximum towing capacity: Up to approximately 8,346 kg for conventional towing.