Drive: 2007 Chevrolet Silverado Heavy Duty and GMC Sierra Heavy Duty
Heavy duty truck engineers like General Motor’s Jim Mikulec, the man responsible for component integration in the 2007 Chevrolet Silverado and GMC Sierra HD pickups, must be feeling like they’re on a treadmill where each revolution spins a little faster than the last.
Since 2000, when the GMT 800 heavies were introduced, tougher competition and stricter government regulation have forced GM’s truck wizards to continuously refine and improve their product - especially the Duramax diesel engine that’s in its fourth iteration since we first drove it.
Last week we were in San Diego driving the all new GMT 900 HDs. These trucks ship with standard 6-speed transmissions and a choice of new 6.0-liter gasoline or 6.6-liter diesel powerplants. Last year's 8.1-liter big block V8 is no longer offered. Gone, too, is the option of a manual gearbox.
GM supplied 26 different Sierras and Silverado for us to spend time in, demonstrating nearly every kind of cab, bed, engine, payload, and towing configuration you could come up with for these workhorse pickups.
We drove at least 30 minutes and 10 miles in 6 of these trucks through San Diego County. The terrain almost matched the variety of the trucks. It ranged from sea level to 5,500 feet over SoCal super highways and two lane twisties through the high desert.
First up, a silver GMC Sierra 2500HD 4WD extended cab long bed powered by the new Gen IV Vortec 6.0-liter gasoline V8. It was ballasted with a simulated 2,575 pounds of payload in the box.
The Sierra’s curb weight is 3 tons, and its gross vehicle weight rating (GVWR) is 9,200 pounds, so we were about 600 pounds shy of the max GVWR. Even though the new Vortec is rated a best in class 353 horsepower and 373 lb-feet, the driver could still feel the burdened response of the engine against the weighty payload, especially during launch through 3rd gear when the broad range of the new 6L90 6-speed transmission became a practical advantage.
The 6L90 gearbox is lighter and smaller than the 4-speed 4L80 heavy duty transmission in the outgoing HDs.
There are two rear axle choices for the 6.0-liter, 3.73 and 4.10. We were equipped with the stouter 3.73 axle, so no doubt this also contributed to some of the strain during initial acceleration.
Equipped with the standard Z85 suspension package, the handling and steering of the truck proved extremely capable managing the near full load. While not necessarily sports car like, steering was very precise and driver feedback near perfect. This was especially noticeable and helpful on the surface streets that wind through San Diego’s urban and hilly neighborhoods.
Both 2WD and 4WD GMT 900 heavy duties use recirculating ball steering with a ratio of 24:1. This is nearly twice the steering ratio of the 2007 Dodge Ram HD, which offers a 13.4:1 recirculating ball system only on its 4WD pickups.