The Plymouth GTX arrived on the muscle car scene in 1967 as Plymouth s serious contender, a hot rod version of the square Belvedere, with a Hemi V 8 option. Base motor in the GTX was the 375 hp, Super Commando 440 cid V 8, capable of 0 60 mph in 6.8 seconds, with a quarter mile time of 14.6 seconds and a top speed of 121 mph. And that was when the car was equipped with an automatic transmission and not an optional 4 speed. The 426 Hemi option cost extra and only 703 GTX hardtops and 17 convertibles were so equipped in 1967.