Final answer:
The legal speed at which you may drive on a freeway posted for 65 mph is 65 mph. The de jure speed limit is what's legally enforceable, despite the behavior of other drivers.
Step-by-step explanation:
When driving on a freeway posted for 65 mph, the legal speed limit you may drive is 65 mph, regardless of the speed at which other vehicles are traveling. Although there is a certain amount of leeway for minor speedometer inaccuracies, exceeding the posted limit can result in a traffic citation.
Typically, the de jure speed limit is what's legally enforceable, while the de facto speed limit refers to the threshold at which enforcement actions like being pulled over initiate, and this usually coincides with the posted speed limit signs.