2024 GMC Sierra HD AT4X Diesel Takes On World’s Toughest Towing Test

This past year, GMC introduced the Sierra HD AT4X, its first off-road-focused HD pickup truck. Then, they went one step further with the SierraHD AT4X AEV Edition. With additional off-road components from American Expedition Vehicles, such as heavy-duty skid plates and stamped-steel powder-coated bumpers with a design that increases approach and departure angles, this edition really stands apart in the category.

The experts at The Fast Lane’s dedicated truck site TFLTruck had already tested the model’s extensive off-road capabilities. TFLTruck then pondered another point: since one of the points of an HD truck is increased towing capacity, is the SierraHD AT4X AEV Edition still a suitable towing vehicle? Or has it sacrificed towing ability with all of its off-road hardware, including its 35-inch tires? On paper, you have something of an answer, as every pound you add to the truck theoretically reduces towing capacity by the same amount, particularly if the added pounds aren’t related to towing enhancement. That still leaves this truck with an 18,400-pound maximum towing rating. TFLTruck wasn’t interested in testing its maximum towing capacity. They just wanted to know how well this off-road-focused behemoth can tow a reasonably heavy load through what they call the “world’s greatest towing test”. Specifically, they hooked up the Sierra HD 2500 AT4X AEV Edition to a trailer holding a CAT skid-steer loader amounting to about 11,000 pounds and drove it up and down what they call the Ike Gauntlet.

Named after the Eisenhower tunnel within it, what TFLTruck dubs the Ike Gauntlet is an 8-mile portion of the I-70 interstate that crosses the continental divide in the middle of the Colorado Rocky Mountains. It has a 7% grade with a total elevation of 11,158 feet above sea level. As TFLTruck states: “If a pickup truck does well here, it will tow your trailer with ultimate confidence anywhere.”

The HD 2500 AT4X AEV Edition is 4WD only and available with either the 401-horsepower 6.6-liter gas V8 or the 6.6-liter DuraMax turbo-diesel V8. As this is a towing test, they chose a truck with the Duramax, which is rated at 470 horsepower with a whopping 975 lb-ft of torque. Controlling the engine’s revs is the job of an Allison-branded 10-speed automatic transmission. TFLTruck used the same Gen-Y-branded hitch they use on all their towing tests, which was helpful in bringing the connection down to a proper level from the truck’s elevated ride height. 

No Drama Uphill

As mentioned, the trailer was nowhere close to the truck’s maximum towing capacity. The truck had just two occupants and no additional payload in the back so the 6.6-liter Duramax had no trouble pulling the trailer up the 7% incline or dealing with the 5-figure elevation levels. To quote: “This GMC Sierra HD was confident, stable, and quiet on the way up the mountain.”

Steady Downhill

Heading downhill, power is not an issue, but control is. They made the descent with the cruise control set to 50 and with the adaptive setting defeated. The truck’s Tow-Haul mode was set, as was the exhaust brake. As set, the brakes were only applied once, and that was near the beginning of the descent. The exhaust brake did the rest to keep the truck and trailer at a steady 50 mph. They were able to monitor this by the Trailer Brake Gain digital readout that appears on the HD’s driver information panel. 

Confidence on Any Grade – In the Ike Gauntlet, the 6.6L Duramax-powered Sierra 2500HD AT4X outperformed Ford Super Duty in downhill brake control and stability—delivering strength you can trust at 11,000+ feet.

The Pickup that Can Do Everything

This was a phrase frequently used during the test. Having already tested its extensive off-road ability and its accommodations for regular truck use. This appeared to be the final test to the question: “Can one pickup really do everything?” With a flawless performance on their “World’s toughest towing test”, the answer appears to be an unequivocal “Yes!”. There is a price to be paid for all the hardware, of course. The Sierra HD 2500 AT4X AEV Edition, as tested, runs north of the 6-figure threshold, as does the model Valley Buick GMC has in stock. But if you want one pickup truck that can do everything, it is the one to get.