How fast can a 1970 Plymouth gtx go?

The top speed of a 1970 Plymouth GTX varied significantly depending on the engine and options. There's no single answer. However, a well-maintained example with the top-of-the-line engine (the 440 Six Pack) could likely reach speeds in the 130-140 mph range. Keep in mind that this is a theoretical maximum under ideal conditions and wasn't a figure officially tested or published by Plymouth. Lower-spec engines would have considerably lower top speeds. Factors like tire condition, gearing, and even air density also play a significant role.