to replace the conventional swinging panel doors. The endgate included a hidden manual crank handle on the exterior for lowering the rear glass. Endgates could be ordered with an electrically operated window regulator to replace the exterior crank, which used a key to operate. Once the rear glass was down, the endgate became a pickup-style tailgate that folded down for cargo loading. Rear floor heat and ceiling air conditioning systems were optional. The standard fuel tank could be replaced with (from 1973) or a (from 1974). For model year 1981, automatic locking front hubs were added for shift-on-the-move four-wheel drive models, and the NP208 transfer case replaced the NP205 in most models. For 1984, asbestos was removed from rear brakes. For 1985, a new grille was used. A total of 64,670 Suburbans were made in 1985. For model year 1987, the method of fuel delivery for the engines was switched from carburetors to electronic fuel injection. (However, for the 454 cubic inch or 7.4-liter displacement engine, carburetors were still made available along with electronic fuel injection for the 1987–1989 model years). The system that GM chose was called throttle body injection, or TBI. Suburban gained rear-wheel anti-lock braking system (ABS) for the 1990 model year. A heavy-duty four-speed automatic transmission, the 4L80-E was added for 1991. Also for 1989, the grille was redesigned to accommodate a quad side-by-side headlight setup and larger marker lights/turn signals than the previous dual filament single headlight setup, which was styled concurrently with the GMT400 light duty trucks. Economy models (usually for fleet usage) were available with only two headlights (again, based on the base model C/K series). GM temporarily changed the usual "C/K" designation to "R" and "V" for the 1987 through 1991 model years. This was done to avoid confusion with the GMT400-based Chevrolet C/K pickup trucks, which were introduced in 1987, during the overlap period. Eighth generation (1992) The GMT400-based Suburbans were introduced in December 1991 for the 1992 model year. The C/K pickup had already switched to the newer platform in 1987 for the 1988 model year. Both 2WD and 4WD models, designated "C" and "K", were offered, as well as half-ton and three-quarter-ton ("1500" and "2500") models. The base engine for all variants was the small-block 5.7 liter V8. The big-block 7.4-liter V8 was optional for the 2500 series. The optional 6.5 L Turbo diesel was available on all models