were the first generation of the C/K to be designed with the use of computers and wind tunnels, optimizing the exterior shape for lower drag and improved fuel economy. The chassis was an all-new design (with all trucks receiving a leaf-spring rear suspension); K-Series trucks moved to all-wheel drive (shift-on-the-fly 4x4 was introduced for 1981). Alongside the introduction of the four-door crew cab, the third generation C/K marked the introduction of a dual rear-wheel pickup truck ("Big Dooley"). For 1978, the C/K became the first American full-size pickup truck sold with a diesel engine (a 5.7 L Oldsmobile diesel V8); a 6.2 L V8 diesel was introduced for 1982. This generation also marks the first use of the Chevrolet Silverado nameplate (in use for Chevrolet full-size trucks today). The Rounded Line generation is the longest-produced version of the C/K model line, produced for 18 model years. For 1987, it was renamed the R/V series (to accommodate the fourth-generation C/K marketed alongside it) and was gradually phased out through the 1991 model year. Fourth generation (1988–2002) The fourth-generation C/K was introduced in April 1987 for the 1988 model year. The first line not assigned a generational moniker by General Motors in nearly three decades, it is known by its GMT400 internal codename. Slightly smaller in cab dimensions than the Rounded Line generation, the fourth-generation C/K increased interior space over its predecessor. After trailing Ford and Dodge by over a decade, GM introduced the C/K in an extended-cab configuration. For nearly five years, the fourth-generation C/K was sold alongside its R/V series predecessor, as the crew cab pickup (which served as the basis of the Suburban SUV) was not released until the 1992 model year. In a branding change, the C/K nomenclature became exclusive to Chevrolet, as all GMC pickups became Sierras (GMC retained the C/K nomenclature for its internal model codes). Chevrolet introduced several specialized variants of the C/K series, including the work-oriented W/T 1500, off-road Z71, and the high-performance 454SS. Between one-ton trucks and medium-duty trucks, Chevrolet and GMC offered the C3500HD chassis cab for commercial use. For 1995, the fourth-generation C/K underwent a mid-cycle revision, adding a driver-side airbag (dual airbags became standard for 1998). For 1996, the extended cab was redesigned, adding a rear-hinged passenger-side third door. For the 1999 model year, to accommodate the introduction of the GMT800-chassis Chevrolet Silverado/GMC Sierra, the fourth-generation C/K was renamed as