unit that ran the full length of the roof had been available since 1967). The Comfort-Tilt steering wheel became optional in 1971. This generation of Suburban coincided with the rapid growth of the recreational vehicle market. While only about 6,200 Suburbans were produced in 1967, by 1972 that number had grown to some 27,000. Seventh generation (1973) With the third generation Rounded-Line C/R & K/V models, the Suburban became a four-door vehicle. The Rounded-Line 1970s body style remained largely unchanged for 19 model years – making this series the longest Suburban generation in production. 2WD (C/R) and 4WD models (K/V) were both available in 1/2 and 3/4 ton ("10" and "20") chassis. Suburbans for model year 1973 now had two doors on each side (the previous generation had just one door on the driver's side), front-and-rear air conditioning, a baggage rack, a heater under the third seat and step-plates for easier access. A new Eaton Automatic Differential Lock was introduced as an option extra for the rear differential. For the 1973 model year, the base engine was the 250 CID inline-six (100 net HP), with a 307 or 350-cubic-inch small-block V8 (115 or 155 net), or the new-for-1973 454 CID big-block V8 (240 net HP) optional. The 307 V8 was dropped for 1974, supplanted by the 305 and 400 small block V8's in 1976. The 400 was dropped after 1980, leaving the 350 as the only available engine in K-series Suburbans as the 454 was not yet offered in 4x4's. In 1982 a new Detroit Diesel V8 engine was available, producing and . of torque. The Diesel later became the engine of choice for Suburbans exported to Europe from USA. Except for the discontinuation of the 305 V8 in 1988, the engine line-up continued mostly unchanged, with the 350, 454, and 6.2 diesel, until the Suburban was redesigned onto the GMT400 chassis in 1992. Three-speed Turbo Hydra-Matic 350 and 400 automatic transmissions were available, and the four-speed Turbo Hydra-Matic 700R4 was introduced in 1981 and was available with the small block and 6.2-liter diesel. Towing packages, with lower axle ratio and heavy-duty cooling additions, were optional. Trim options included the base model ("Custom Deluxe" before 1975, "Scottsdale" or "High Sierra" after) and the upgraded ("Cheyenne Super" before 1975, "Silverado" or "Sierra Classic" after) versions. Optional second and third-row bench seats allowed for nine-passenger configurations. A popular "endgate" option was offered