2015 Chevy Silverado HD and GMC Sierra HD unveiled at State Fair of Texas

Chevrolet and GMC unveiled new, 2015 heavy duty versions of their recently introduced Silverado and Sierra at -- where else? -- the Texas State Fair. Updates include new exterior and interior styling, new engines and a host of new options.

Chevy and GMC say the new exterior shapes are more aerodynamic, thanks to new inlaid doors fitting into the body sides that also help reduce wind noise. GM sweated more aero fine points such as tuning body details in the roof and tailgate spoiler in the wind tunnel, also hoping to reduce wind noise and improving aero.

GM says it knows HD pickups are often used for towing in adverse conditions so they are testing the bejeesus out of the trucks: Before they're released to dealers, the trucks will undergo more than 6 million miles (240 trips around the earth) of durability testing in the torture chambers at GM's Milford Proving Ground, over deserts and mountains of Nevada and across the frozen flats of Kapuskasing, Ontario. In addition, the trucks will accumulate more than 7 million real-world miles during final testing, bringing total test miles to more than 13 million.

Engineers are paying particular attention to airflow for engine cooling, testing cooling airflow in the wind tunnel and on the road. The idea is to make sure engines could maintain power on long upgrades at high temperatures.

Towing was also a priority in development, refining cruise control, auto-grade braking, and diesel-exhaust brake; all combine to help maintain a set speed up and down grades, even when towing a heavy trailer in the mountains. The customer sets the desired speed and the truck will do the rest -- increasing or decreasing power, downshifting to increase engine braking, and engaging the exhaust brake on the diesel engine. Auto-grade braking is activated by pressing and holding the tow/haul button in the shifter for three seconds. The trans will automatically downshift when needed to enable engine braking to help slow things down.

There's also a diesel-exhaust brake: On Duramax diesel models it uses the turbocharger's variable vanes to create backpressure to help slow the truck even on long, steep downgrades. Additionally StabiliTrak with Trailer Sway Control is now standard on all Silverado HDs, including 3500 dually models, providing additional security to the drive.

Of course the new pickups ride on fully boxed frames with large cross sections, hydroforming and high-strength steels. The independent front suspension has been engineered for heavy duty use with forged steel upper control arms and precision-machined cast iron lower control arms. Torsion bars are tailored to the trucks' various weight ratings allowing easier trim height adjustment to account for the weight of say, a snow plow or other accessories. Rear springs are asymmetrical leafs. Payload and towing are both up compared to 2014 models: Maximum payload is 7,374 pounds, the best for any HD pickup, GM says, up from 7,222 pounds; maximum available conventional towing is 19,600 pounds with the available factory hitch, up from 18,000 pounds and also best in class, according to GM. Maximum available fifth-wheel towing is 23,200 pounds, up from 22,500 pounds.

There are more than 150 combinations of cabs, bed lengths, powertrains, drivetrains and axle ratios, so you can spec exactly the right truck for your towing and hauling needs.

The new HDs get the same interior updates, cab structure, and door designs as the standard Silverado and Sierra. The dash, instrument cluster and multiple storage options are tailored for truck customers' needs. For example, controls are designed to be easy to use even with work gloves on.

The new cab is stronger, with nearly two-thirds of the structure made from high-strength steel. Crew cab models' rear doors are larger, and combined with new rear seats, create about 2 inches more rear legroom, and are designed for easier entry and exit for back-seat passengers.

A new double cab model with forward-hinged rear doors replaces the extended cab model, also for easier access to the back seat.

Power sources include the Duramax 6.6-liter turbodiesel and Allison 1000 six-speed automatic transmission. That combo will continue to be available on both 2500 and 3500 Silverados. Rated at 397 hp and a whopping 765 lb-ft of torque, the diesel features piezo-actuated fuel injectors for improved performance and efficiency and the aforementioned variable vane turbocharger.

The standard engine on all HDs is a 6.0-liter Vortec gas V8 with continuously variable valve timing and a six-speed automatic transmission. The Vortec cranks out 360 hp and 380 lb-ft of torque on 2500 models, 322 hp on 3500s. A bi-fuel version of the 6.0-liter V8 with special hardened valves and valve seats for operation on gas or compressed natural gas will be available.

Other options include forward-collision alert, lane-departure warning and front- and rear-park assist. Head-curtain and seat-mounted side airbags are standard on 2500 and available on 3500 models. You can also order up 4- or 6-inch assist steps in chrome or black, 20-inch wheels, body-side moldings, and a bunch of chrome accents.

The trucks will be in showrooms early next year; pricing will be announced at a later date, but GM representatives confirm that MSRPs will remain essentially flat from 2013.

By Wes Raynal