In 1991 GM introduced a 15,000 pound GVWR truck 3500HD under the Chevrolet and GMC nameplates. It was marketed as a truck to bridge the gap between light duty trucks “pickup trucks” and medium duty trucks.
The 3500HD was only offered as a standard chassis cab until 1996 when a crew chassis cab was also offered. Upper cab marker/clearance lights were not optional equipment on the 3500HD. The common drive axle used on the 3500HD was the Dana 80, an 85.8″ wide full floating axle with an 11 inch ring gear fitted with 19.5″ x 6.0 tires. The front axle was a solid I-beam drop axle, similar to the axles of medium and heavy duty trucks. Both front and rear leaf sprung axles had disc brakes.
Available wheelbases were; 135.5″, 159.5″, and 183.5″. The 3500HD frames are very different from the C/K3500 cab and chassis. The HD frame is much heavier and exits straight out behind the high mounted cab necessitating the unmistakable HD filler panel between the bumper and grille. The front fenders were also equipped with the same flares used on 4×4 models of the lighter trucks to cover the increased track width and larger tires.
Two transmissions were offered in the 3500HD; the 4L80E 4spd OD automatic, and the NV4500 5spd manual. Engine offerings for the 3500HD included three gasoline engines; from 1991-1995 5.7L 350 Small Block and 7.4L GEN V Big Block.
In 1996 both the 5.7 and GEN V 7.4 were replaced by the new Gen VI 7.4L Vortec Big Block. The RPOL65 6.5L Turbo Diesel debuted in the 3500HD in 1992, the year of the engine’s release. The 6.5 was the only diesel engine offered for the entire production run of the T400 3500HD. No diesel was available for 1991, the first 3500HD production year.