* The base engine was slower. A six-cylinder or a smaller V8 would have resulted in a much slower car.
* High-performance V8s were much faster. The top-performing engines, like the 440-6bbl and the legendary 426 Hemi, could achieve 0-60 mph times in the low to mid 6-second range and have a quarter-mile time in the low 13s. However, these numbers vary depending on the condition of the specific car, tire condition, and driver skill.
Therefore, there's no single answer to how "quick" a 1971 Plymouth GTX is. To get a precise speed estimate, you'd need to know the specific engine and drivetrain of the car in question.
