Medium and heavy commercial vehicles aren't changed every four or five years the way passenger cars are. If a medium-duty truck line meets the needs of its customers, manufacturers tend to continue building it as ong as the orders come in.

Orders for General Motors' C-Class medium-duty trucks haven't ceased, by a long shot. But GM has gone ahead and launched its replacement anyway, bringing the medium-duty class into the new century.

The new line is known as the GMT560, and is offered as both Chevrolet and GMC models, the differences mainly being in trim and the dealership that delivers the vehicle. Models will be offered with GVWs ranging from Class 4 through Class 8, to cover every segment. The launch will begin with 2WD regular-cab models, being followed shortly by crew-cab and 4WD versions.

An All-New Look

Anyone familiar with the look of the C-Class trucks doesn't need more than seconds to realize that the GMT560 is different. Its aerodynamic front end, steeply slanted windshield and body-mounted exterior mirrors are an immediate give-away that this is something new.

According to Steve Matsil, chief program engineer for the GMT560, "The GMT560 trucks are designed and built as dedicated medium-duty vehicles that are so user-friendly, it's like you're driving a light-duty vehicle." Matsil added that the GMT560 is the most-researched line of commercial vehicles ever built by General Motors, with available options and flexibility in spec'ing to meet every need.

Exterior hardware such as door hinges and handles has been tested to 500,000 full-load duty cycles to ensure durability.

Inside, the new cabs, offered in regular and crew-cab models, provide improved forward visibility, through frame modifications that allow the radiator and engine to be mounted lower and farther to the rear. This in turn permits a more steeply-sloping and aerodynamic engine hood, allowing the driver to see the ground closer to the front bumper.

The windshield itself is 25 percent larger than that on the previous model, and is wiped by "overlap" type wipers, which clear a larger area of the glass. In applications that allow a rear window, that has increased glass area as well.

Regular cabs offer seating for one, two, or three passengers, while crew cab models can provide seating for up to six people. Driver and front passenger positions can be equipped with air-suspension seating, or a two-person front passenger seat can be fitted. The rear seat in crew cab models is a bench seat that can accommodate three 95th percentile adults, defined as 6 foot, 2 inches and weighing 200 lbs., in comfort, or the seat can be deleted so that the area can be used for secure storage.

Driver's-side airbags are standard, while front passenger airbags are optional. Increased sound insulation and isolation has given the new cabs a 50-percent reduction in noise, vibration, and harshness over the previous model, reducing a major contribution to driver fatigue.

The new instrument panel allows for the fitting of up to 24 warning lights and gauges within easy view of the driver. There is also an auxiliary panel to accommodate accessory controls and gauges. The electrical system has been upgraded to allow more electronic controls and has been given increased reliability and durability through the use of multiplexing to reduce the number of wires, relays, and splices.

Power for Any Purpose

GMT560 C4500 and C5500 series are offered with two newly available engines, the Duramax 6600 turbodiesel V-8 and the Vortec 8100 gasoline V-8. Each engine carries a choice of two power ratings so that fleet users can match power and fuel economy needs to the job at hand. The Duramax 6600 is available with 210 horsepower and 510 lb.-ft. of torque, or 300 horsepower and 520 lb.-ft. or torque. Choices for the Vortec 8100 are 225 horsepower with 350 lb.-ft. or torque, or 325 horsepower with 450 lb.-ft. of torque.

The C6500, C7500, and C8500 trucks can be ordered with a choice of three diesel engines, the Duramax 7200 and 7800, or a Caterpillar 3126E. The Duramax engines have a wide range of power ratings, ranging from 175 horsepower and 400 lb.-ft. of torque to 275 horsepower with 860 lb.-ft. of torque. The Caterpillar can be ordered with power ratings from 190 horsepower and 420 lb.-ft. to 300 horsepower and 860 lb.-ft. of torque. The Vortec 8100 is also available in these classes of trucks, where optimum fuel economy is required, or where a conversion for alternative fuel use (LPG) is mandated.

Transmission choices are equally flexible, including Allison 1000 series five-speed automatics on the C4500 and C5500 series, and Allison 2000 series transmissions in the C6500 and C7500, while the Allison MD3000 is standard on C8500 models with single or tandem axles.

Manual transmissions include a ZF six-speed offered withthe Duramax 6600 and Vortec 8100 engines in the C4500 and C5500 trucks. In the heavier configurations, C6500, C7500, and C8500, Eaton 5- 6- , 9- , and 10-speed transmissions are available, along with two TTC 7-speed models.

Ride & Handling Improved

The GMT560 trucks build on the ride, handling, and braking characteristics of the C-Class trucks. A choice of taper leaf spring, multi-leaf spring, or air suspensions is offered.

Hydraulic braking systems have also been upgraded for easier stopping effort and shorter stopping distances. Four-wheel disc brakes are fitted with a four-wheel, four channel anti-lock (ABS) system, which includes Electronic Brake Distribution and can be combined with an optional traction control system.

Framing it All Up

High-strength 50,000- and 80,000 psi steel is used for the basic construction of GMT560 frames. Frame rails are straight C-channel beams, without any bends or tapers that could cause weak points. Longer frame rails accommodate longer wheelbases, larger bodies, and greater load-carrying capabilities without the need to add any extra reinforcement. A clean back-of-cab design allows GMT560 models to meet National Truck Equipment Association requirements for body and equipment installation.