The Plymouth GTX (1967-1971) was introduced as the Belvedere GTX in 1967 by the Plymouth division to be a "gentleman's" muscle car.
In 1968, after one year of production, the GTX was given a completely new look. A new hour glass body replaced the more box like body of 1967. The Road Runner was introduced by Plymouth as a performance version of the Satellite. There were major changes made in the design of all the Plymouth B-bodies. The Road Runner's base engine was the new 383 cu in (6.3 L) "Super Commando" V8 (renamed the "Road Runner 383"), while the high performance 440 was still standard in the GTX. The TorqueFlite automatic transmission was standard on the GTX, with it being a US $206 option in the Road Runner. The GTX was still based on the Satellite and was offered in two models, the 2-door convertible and the 2-door hardtop. The Road Runner was based on the Belvedere, and had less insulation and comfort items(padding, vinyl roof, trim), which reduced weight, helped produce better track times, and kept it in the low price field.
Source: Wikipedia, 2011