The new-for-2007 GMC Sierra range includes the Heavy Duty models, and the 2500HD is the first of them. Nothing has changed in the year since launch, so and two hard-working engines remain.

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The standard, gas powerplant is a Vortec 6.0 liter V8 that is rated at 353 horsepower at 5,400 rpm and 373 lb-ft of torque at 4,400 rpm. The diesel-fueled alternative is a 6.6 liter V8 labeled Duramax, which is built for GM by Isuzu. This oil-burner makes 365 horses at 3,200 rpm and a stump-pulling and best in-class 660 lb-ft at just 1,600 rpm.

The gas engine comes only with a heavy duty six-speed automatic transmission, which includes a tow / haul mode. The Duramax is hitched exclusively to an Allison 1000 six-speed, which also has a tow / haul mode but throws in electronic engine grade braking. Both combinations get a 3.73-ratio rear axle, with a 4.10 axle available for the gasoline-powered 2500HDs only. All models feature an auxiliary external transmission oil cooler, as well as an external engine oil cooler.

Braking is accomplished with discs all around, with anti-lock control across the range.

There are the five unique cab layout / box length combinations. The Regular cab is available with a long box only, the Extended cab adds a standard box option, and the conventionally-hinged four-door Crew cab also offers both. Maximum payload capacities are the same regardless of engine choice, at 3,892 lbs in the Regular cab 2WD model. Opting for four wheel drive decreases the capacity to 3,646 lbs in the same truck.

As you'd imagine, the diesel is the engine to have if there is lots of towing to be done. When properly equipped, it is capable of towing 13,000 lbs on a conventional trailer, while the maximum fifth-wheel towing capacity is 15,800 lbs (Regular Cab, Long Box, 2WD).

You can build a petrol-powered 2500HD that's just 100 lbs shy of that number, however, while the lowest mass a 6.0 liter will tow is 9,600 lbs.

Equip either truck with a fifth-wheel hitch and the maximums jump to 15,800 lbs for the Duramax and 12,900 for the Vortec.