* Axle Ratio: Different axle ratios were available, affecting the speed at a given engine RPM. A lower ratio (numerically higher, e.g., 4.10) would result in a lower top speed than a higher ratio (numerically lower, e.g., 3.50).
* Tire Size: Larger tires (taller sidewalls) would lower the top speed.
* Vehicle Condition: The overall condition of the engine, transmission, and drivetrain significantly impacted performance. A well-maintained Bronco would be faster than one that's neglected.
* Aerodynamics: While not a huge factor with a Bronco, wind resistance would still affect top speed.
* Load: A heavier Bronco carrying passengers and cargo would have a lower top speed.
While it's impossible to give a precise number, a reasonable estimate for the top speed of a well-maintained 1981 Ford Bronco with a 133 hp engine under ideal conditions would likely be in the range of 65-75 mph. However, it could easily be lower depending on the factors listed above.
