* Specific engine configuration: Even within the 324 hp range (likely referring to the 390 cubic inch big-block V8), variations in carburetor, exhaust, and other components would affect performance.
* Gear ratios: The rear-end gear ratio significantly influences top speed. A lower numerical gear ratio (e.g., 2.75:1) allows for higher top speed than a higher ratio (e.g., 3.50:1).
* Aerodynamics: Minor modifications to the car, or even just the condition of the body (e.g., alignment and wear), could subtly alter aerodynamic drag.
* Road conditions and driver skill: Wind, road grade, and driver skill all play a role.
While some sources might claim a specific number, it's likely an estimate or based on testing under ideal conditions. A reasonable *estimate* for a well-maintained, unmodified 1968 Mustang with a 324 hp 390 engine might be in the 120-130 mph range, but this is highly variable and shouldn't be taken as precise. Reaching those speeds would also require a long, flat, straight road and safe driving conditions.
