Final answer:
The posted speed limit of 55 mph indicates the maximum legal speed under ideal conditions; however, drivers must adjust speed based on road, traffic, and weather conditions. There is some tolerance above the limit, but exceeding it markedly can lead to a traffic ticket.
Step-by-step explanation:
A posted speed limit of 55 mph does not mean you can always drive at that speed under all conditions. The speed limit is a de jure, or legally defined, maximum speed under ideal driving conditions. Factors such as weather, traffic, and road conditions may require drivers to go slower to maintain safety. It's essential to adjust your driving speed to the current conditions, even if that means going below the posted limit. The idea of a de facto speed limit suggests there may be some leeway around the exact limit; however, consistently exceeding the speed limit can lead to a traffic ticket. In different contexts, speed is measured differently, and keeping an eye on the conversion between kilometers and miles per hour is crucial when traveling in countries using the metric system. The fuel efficiency of the car also varies with speed and is optimal around 45-50 mph, which is one reason why during the 1970s oil crisis in the U.S., highway speed limits were set at about 55 mph.