The sixth-generation Ford F-Series was introduced for the 1973 model year. Known by enthusiasts as the "Dentside," the F-150 model series, which debuted for the 1975 model year, was introduced between the F-100 and the F-250. The F-150 is referred to as a "heavy-duty" F-100. To avoid certain emission control restrictions, it featured a larger payload from the F-100. While all 6th Gen F-Series trucks were fitted with catalytic converters, the F-150 was not. The F-150 was based on the 1/2 ton chassis, whereas the F-250 was based on the 3/4 ton chassis. The F-150 featured heavier duty axles and springs. It would ultimately become the best-selling vehicle in the US. It could be configured as a regular cab and a two-door Supercab with the option of a "Styleside" or "Flareside" bed as a long bed. Short beds were not offered on the F-150. Both beds and cabs could be optioned as a two-wheel drive or four-wheel drive drivetrain. Engines offered were a six-cylinder 300cid and four V8's throughout the years, 302cid, 351cid, 400cid, and 460cid. Transmissions offered were automatic, three-speed, and, later, 4-speed manual depending on engine and drivetrain. All models could be outfitted with any three available trim levels, including the entry-level Custom, mid-level Ranger, or top-of-the-line Ranger XLT and Ranger Lariat. For 1978, square headlights replaced the previous models' round ones on higher trim package models, such as Lariat and Ranger, and in 1979 became standard equipment. 1979 would also be the last year for Sixth Generation production, with the Seventh Generation F-Series replacing it.